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 ESRI's SDE 
or Oracle's Spatial extension. 
PostGIS follows the OpenGIS "Simple Features Specification for SQL" and 
will be submitted for conformance testing at version 1.0.

PostGIS has been developed by Refractions Research Inc as a research project 
in open source spatial database technology. PostGIS is released under the 
GNU General Public License. We intend to continue development as time and 
resources permit. Our list of future projects includes enhanced technology 
for data loading and dumping, user interface tools for direct data access 
and manipulation, and support for advanced topologies at the server side, 
such as coverages, networks, and surfaces.

WWW: http://postgis.refractions.net/