What is GIS?
What Is GIS?
Open Source GIS Software

FlowMap is a freeware application designed to analyze and display flow data. This application was developed at the Faculty of Geographical Sciences of the Utrecht University in the Netherlands.
Platforms: Windows OS

GMT Mapping Tools
GMT is a free, public-domain collection of ~60 UNIX tools that allow users to manipulate (x,y) and (x,y,z) data sets (including filtering, trend fitting, gridding, projecting, etc.) and produce Encapsulated PostScript File (EPS) illustrations ranging from simple x-y plots through contour maps to artificially illuminated surfaces and 3-D perspective views in black and white, gray tone, hachure patterns, and 24-bit color.
Platforms: UNIX, Macintosh

Geographic Resources Analysis Support System (GRASS) is the public domain GIS software application originally developed by the US Government. GRASS  is probably the most well-known open source and original GIS software applications. GRASS is a raster-based GIS, vector GIS, image processing system, graphics production system, data management system, and spatial modelling system. GRASS can be downloaded for free.
Platforms: Linux, Macintosh, Sun Solaris, Silicon Graphics Irix, HP-UX, DEC-Alpha, and Windows OS

gvSIG is an open source GIS application written in Java.
Platforms: Windows, Macintosh, Linux, UNIX

MapWindow GIS
MapWindow GIS is open source GIS application that can be extended through plugins.  The application is built using Microsoft’s .NET
Platforms: Windows

OpenJUMP GIS is an open source GIS written in Java through a collaborative effort by volunteers.  Formerly known as JUMP GIS, the application can read shapefiles and GML format files.
Platforms: Windows, Macintosh, Linux, UNIX

Quantum GIS 
Also referred to as QGIS, Quantum GIS is an Open Source Geographic Information System (GIS).
Platforms: Linux, Unix, Mac OSX, and Windows.

SPRING is a GIS and Remote Sensing Image Processing system with an object-oriented data model which provides for the integration of raster and vector data representations in a single environment.
Platform: Windows, Linux, UNIX, Macintosh

TNTLite MicroImages, Inc. provides TNTlite as a free version of TNTmips , the professional software for geospatial data analysis. The free TNTlite product has all the features of the professional version, except TNTlite limits the size of Project File objects, and TNTlite enables data sharing only with other copies of TNTlite (export processes are disabled). Can either be downloaded or ordered on CD.
Platforms: Windows

uDig GIS
uDig GIS is a free, open source GIS desktop application that runs on Windows, Linux and MacOS. uDig was designed to use OGC’s OpenGIS standards such as WMS, WFS and more. One-click install allows you to view local shapefiles, remote WMS services and even directly edit your own spatial database geometries.
Platforms: Windows, Linux, Macintosh

Open Source Web Mapping

Written in java, GeoMajas is an open source GIS framework for the web.

Java based open source server software that allows users to edit and share geospatial data and uses open standards to spublish GIS data.

MapGuide Open Source
First introduced as open source by Autodesk in 2005, MapGuide Open Source  allows for the development of web based mapping.

An open source mapping development framework for web mapping applications based on the Pylon Pythons web framework.

MapServer is an Open Source development environment for building spatially enabled Internet applications. The software builds upon other popular Open Source or freeware systems like Shapelib, FreeType, Proj.4, libTIFF, Perl and others.

Javascript library that is open source for displaying GIS data within a browser environment.   OpenStreetMap uses OpenLayers for its main map display (aka the "Slippy Map").

Built on open source libraries (Mapniknode.jsbackbone.js,express and CodeMirror).  The Chicago Tribune included TileMill in a series entitled Making Maps using PostGIS, Mapnik, TileMill, and Google Maps.

Open Source GIS Components and Packages

EDBS Reader
A free (GPLed) reader software for the EDBS format has been released: EDBS_extra 2.0. This open source utility is written in 'C'. The page is mostly in German.

GIS/RS application for Linux and Gnome platforms. Open source code is available for downloading from this site.

GeoTools is an open source, Java GIS toolkit for developing standards compliant solutions. It's modular architecture allows extra functionality to be easily incorporated. GeoTools aims to support OpenGIS and other relevant standards as they are developed.

MITAB is an Open Source (i.e. Free) C++ library to read and write MapInfo .TAB (binary) and .MIF/MID files. It is based on the OGR library which is an implementation of the Open GIS Consortium Simple Feature specification.

OpenEV is a library, and reference application for viewing and analysing raster and vector geospatial data. Download for Windows 98/NT/2000, Linux, Irix or Solaris systems.

OpenMap is a FREE JavaBeans open source software component for viewing spatial data. JavaBeans is a component specification for software written in the Java language. In contrast to other GIS software components which offer both data viewing and analysis capabilities, OpenMap is primarily for data viewing and offers very little in the way of analysis functionality.

rmap is a package that will allow you to generate images of the earth from a distance or fairly zoomed in. The code is a small C binary that reads a datafile of vectors to generate the image.

Tkgeomap is a set of extensions to the Tcl/Tk scripting language for manipulating and displaying geographic data.

Topology Framework .NET  (TF.NET)
TF.NET represents a managed topology manipulation API capable of handling managed objects representation of topological KIGWEBEntities based on other popular APIs, exposing it's JTS-based common topology manipulation core to them. Supported external managed APIs include: OSGeo Feature Data Objects (FDO) geometries, OSGeo MapGuide Server (FDO-based) geometries and Autodesk ObjectARX geometries (a.k.a. KIGWEBEntities) and, most recently, Oracle's ODP.NET.  Functions provided include: Spatial predicates (based on the DE-9IM model), Overlay functions (intersection, difference, union, symmetric difference), Buffer, Convex hull, Area and distance functions, Topological validity checking, Coordinate systems manipulation (transformations), Topological graphs manipulation, and more. TF.NET libraries are free, licensed under GNU LGPL and available for download from Google Code page.

Vhclmaps is a package of map viewers and spatial data servers that work with map databases.

Related Resources:

Also referred to as the Open Source Geospatial Foundation, this entity supports the collaborative development of open source GIS software.

Open Source Imagery Processing
Open source software for processing imagery.

State of Open Source GIS
Intensive survey by Paul Ramsey published in September of 2007 that reviews open source GIS grouped by programming language.  The survey looks at C, Java, .NET and web-based GIS.

Commercial and Proprietary GIS Software

AGIS for Windows is a mapping and simple GIS package specifically designed to be easy to use, and distributed as shareware via the world wide web.
Platforms: Windows

Autodesk has a series of software applications designed to meet GIS needs in a variety of areas that interfaces with their CAD software.
Platforms: Windows

Bentley Systems, Inc.
Bentley provides software for the "Design, construction, and operation of the world's infrastructure". The company’s software serves the geospatial, building, plant, and civil vertical markets in the areas of mapping, architecture, engineering, construction (AEC) and operations. Bentley offers a wide range of products for surveying, GPS, photogrametry, remote sensing, imaging, conversion, mapping, cartography, and other geospatial applications built on MicroStation Products:  Bentley Map – Desktop GIS, Bentley Cadastre – Desktop land management GIS, Bentley Descartes – Desktop image editing, analysis, and processing, Bentley Geo Web Publisher – GIS web publishing and viewing, Bentley PowerMap Field – Field-enabled GIS and redlining.

Commercial software package for Mac OS featuring: Support for a huge number of import formats, including popular Raster formats, manual editing and georeferencing, automatic geocoding, integration with online mapping, output to large-format printers.  Thirty-day demo available for download.
Platforms: Macintosh

DeLorme is the producers of XMap , a GIS application "with 80% of the functionality found in a traditional GIS at 15% of the cost" . Performs such functions as geocoding, image rectification, 3D visualization and coordinate transformation.
Platforms: Windows

Environmental Systems Research Institue has been creating GIS software for over 30 years. Recognized as the leader in GIS software, it's been estimated that about seventy percent of GIS users use Esri products. Esri overhauled their software packages into an interoperable model called ArcGIS (the desktop GIS is referred to as ArcMap).  In addition, Esri has developed plug-ins called extensions which add to the functionality of ArcGIS. Demo and light versions of Esri software are available for downloading. You can also find free data to use with Esri products. Platforms: Windows

IDRISITaiga is "an integrated GIS and Image Processing software solution, provides nearly 300 modules for the analysis and display of digital spatial information." It is developed and distributed on a non-profit basis by the Clark Labs, a project within the Graduate School of Geography at Clark University in Worcester, MA.
Platforms: Windows OS
Further Resources: Idrisi Resources

Intergraph makes several GIS applications . Most of the GIS packages are designed with an Open GIS in mind and therefore can work with a variety of other GIS software formats. Intergraph has developed products that help merge GIS with information technology (IT) and business process improvement tools. Intergraph offers the GeoMedia family of solutions and Modular GIS Environment MGE Suite of mapping and GIS applications.

Using an open architecture, the GeoMedia product suite integrates geospatial information throughout the enterprise and provides the tools needed to develop business-to-business and custom client applications using industry standard development tools. GeoMedia offers uninhibited access to all geospatial data formats without the need for data translations. Currently in Version 4.0 the GeoMedia family is made up of GeoMedia, GeoMedia Professional, GeoMedia WebMap, and GeoMedia WebEnterprise.

The Modular GIS Environment (MGE) product suite provides production-ready capabilities for automating, managing, analyzing, and presenting GIS data, and is completely interoperable with GeoMedia.

Platforms: Windows OS
Further Resources: Intergraph

Manifold System provides comprehensive, professional grade GIS software for $245 that includes a very wide array of features. Manifold imports data from over 80 different GIS formats, including all formats used by Federal government sites for free Internet downloads, and Manifold allows seamless, simultaneous work with vector drawings, raster images, terrain elevations and raster data sets either as 2D displays or 3D terrain visualizations. Manifold includes exceptional DBMS capabilities, full development facilities and includes a built-in Internet Map Server for fast and easy publication of GIS projects to the web without programming. Options include US Street address geocoding and the Enterprise Edition, for centralized geospatial data storage on enterprise servers that can be used by many GIS operators at once. Manifold was the first GIS to attain "designed for XP" status with Microsoft and the Manifold Internet Map Server works perfectly within ASP.NET servers. For info, see http://www.manifold.net/professional.
Platforms: Windows

Ortelius is a "map illustration" software package that adds one more selection to the very limited mapping software options out there for Macintosh users.  A free trial download is available.
Platforms: Macintosh

MapInfo - Pitney Bowes Business Insight (PBBI)
PBBI's flagship software is MapInfo, a suite of GIS software. MapInfo Professional is their leading GIS product containing the most advanced analytical tools. MapInfo also offers plug-ins called add-ons to enhance the functionality of MapInfo Professional. For the development side, MapInfo offers Map-X. Through an Active X component, developers can embed mapping applications into other applications such as Excel. Although it can be used for a variety of analysis, the makers of MapInfo market the software more towards the business sector. Demo versions are available for downloading for some of MapInfo's products.
Platforms: Windows OS

The  Maptitude Mapping Software is a full-featured mapping package for Windows. Designed for ease-of-use, data visualization and geographic analysis, Maptitude comes with comprehensive nationwide and worldwide maps, including complete US street maps, and Census tract and ZIP Code boundaries and demographics. Caliper also produces TransCAD for transportation and logistics. TransCAD is used for solving key analytical problems in transportation planning, management, and operations. TransCAD is used extensively for transportation database development and maintenance, demand forecasting, operations management, and vehicle routing and scheduling.
Platforms: Windows OS

Map Maker
Map Maker Pro is a low-cost and easy to use, yet powerful, map making and Geographical Information System for Windows. It is designed to enable non-expert users to start producing useful maps after only a few hours study. A variety of tools allows you to navigate around a map, measure distances and areas, draw polygons, lines and symbols, and display and edit data.
Platforms: Windows OS

My World GIS is a full-featured GIS designed for educational use. My World provides a carefully selected subset of the features of a professional GIS environment. These features include multiple geographic projections, table and map views of data, distance-measurement tools, buffering and query operations, customizable map display. They have been selected to provide the greatest value to students without overwhelming them with complexity The features are accessed through a supportive interface designed with the needs of students and teachers in mind.  My World can import data from the industry-standard shapefile format, as well as from tab and comma-delimited text files.
Platforms: Windows, Macintosh, Linux, Solaris

SuperGIS Desktop
Desktop GIS solution from SuperGeo for geoprocessing and displaying, editing, managing, querying and analyzing geographic data.
Platforms: Windows

Terrain Tools
Terrain Tools, produced by Softree, is a software package for surveying and mapping. It is ideal for the forester, geologist, surveyor or resource scientist who is not a GIS specialist, but who needs to quickly produce working maps and site plans.

TNT Products
Created by Microimages, The TNT Products is a suite of GIS applications for fully integrated GIS, image processing, CAD, TIN, desktop cartography, and geospatial database management.
Platforms: Windows, UNIX, Macintosh

Free GIS data

Country level GIS Data
Free data from ESRI

Other Useful Informative Sites

GIS Geography