Post GIS adds support for geographic objects to the PostgreSQL objectrelational 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

Website: http://postgis.refractions.net/