PostGIS - Geographic Objects In PostgreSQL
PostGIS "spatially-enables" the open source PostgreSQL database. According to the PostGIS website, it is similar to the role played by ESRI's SDE or Oracle's Spatial extension. PostGIS 0.8.2 was released on May 27th, 2004. It is available under the GNU General Public License (GPL).
There are now several open source tools which work with PostGIS. For internet mapping, the University of Minnesota Mapserver can use PostGIS as a data source. The GeoTools Java GIS toolkit has PostGIS support, as does the GeoServer Web Feature Server. GRASS now supports PostGIS as a data source, through the PostGRASS driver. The JUMP Java desktop GIS viewer has a simple plugin for reading PostGIS data, and the QGIS desktop has good PostGIS support. PostGIS data can be exported to several output GIS formats using the OGR C++ library and commandline tools (and of cource with the bundled Shape file dumper). And of course any language which can work with PostgreSQL can work with PostGIS -- the list includes Perl, PHP, Python, TCL, C, C++, Java, and more.