Mapserver!

GRASS GIS (Geographic Resources Analysis Support System) is an open source, free software (FOSS) Geographical Information System (GIS) with raster, topological, vector, image processing, and visualization functionality. Used for geospatial data management and analysis, image processing, graphics/maps production, spatial modeling, and visualization, GRASS is currently used in academic and commercial settings around the world, as well as by many governmental agencies and environmental consulting companies. »

PostGIS adds support for geographic objects to the PostgreSQL object-relational database. In effect, PostGIS "spatially enables" the PostgreSQL server, allowing it to be used as a backend spatial database for geographic information systems (GIS), much like ESRIs SDE or Oracles Spatial extension. PostGIS follows the OpenGIS "Simple Features Specification for SQL" and has been certified as compliant with the "Types and Functions" profile.

MapBox makes it easy to design beautiful custom maps, and fast to integrate them into websites and mobile applications using Open Street Map data.

ArcGIS is geographic information system (GIS) software for visualizing, managing, creating, and analyzing geographic data. Using ArcGIS, you can understand the geographic context of your data, allowing you to see relationships and identify patterns in new ways.

OpenGeo provides support and services around GeoServer, OpenLayers, PostGIS, GeoWebCache, and GeoExt.The OpenGeo Suite brings together the OpenGeo Architecture into a single, easy-to-install integrated software package. By developing each individual component of the OpenGeo Suite individually, we ensure that the Suite integrates with existing systems—proprietary or open source. It is the fastest way to get your geospatial information on the web, leveraging the power of best-of-breed open source geospatial software. »

gvSIG is a Geographic Information System (GIS), that is, a desktop application designed for capturing, storing, handling, analyzing and deploying any kind of referenced geographic information in order to solve complex management and planning problems.

Mapnik is a Free Toolkit for developing mapping applications. Above all Mapnik is about making beautiful maps. It is easily extensible and suitable for both desktop and web development. - C++/Python GIS toolkit

The reference implementation of the Open Geospatial Consortiums WFS and WMS services, GeoServer is a full featured open source software server written in Java that allows users to query, visualize, combine, edit and publish spatial data over the web.

SAGA - System for Automated Geoscientific Analyses - is a Geographic Information System (GIS) software with immense capabilities for geodata processing and analysis. SAGA is programmed in the object oriented C++ language and supports the implementation of new functions with a very effective Application Programming Interface (API). Functions are organised as modules in framework independent Module Libraries and can be accessed via SAGA’s Graphical User Interface (GUI) or various scripting environments (shell scripts, Python, R, ...). »

uDig is a GIS software program produced by a community led by Canadian-based consulting company Refractions Research. It is based on the Eclipse platform and features full layered open source GIS. It is written in Java and released under GNU Lesser General Public License.

Mapline is the simplest mapping software on the planet! Be empowered with map visualizations from Excel spreadsheet data to optimize logistics, enhance market planning, identify growth opportunities, or mitigate market risks. Create your first map now!

Esri Geoportal Server is a free open source product that enables discovery and use of geospatial resources including datasets, rasters, and Web services.

GeoNode is a web-based application and platform for developing geospatial information systems (GIS) and for deploying spatial data infrastructures (SDI).

The Géoclip interactive mapping solution is proposed by EMC3. Géoclip permits to publish online thematic atlas or territorial observatories. Géoclip allows you to build web mapping applications for cartographic representations of statistical data.

The Map Kit framework provides an interface for embedding maps directly into your own windows and views. This framework also provides support for annotating the map, adding overlays, and performing reverse-geocoding lookups to determine placemark information for a given map coordinate.