Sophisticated open-source Object-Relational DBMS supporting almost all SQL constructs, including subselects, transactions, and user-defined types and functions. Completely free database http://www.postgreSQL.org/
Contains community contributed documentation, CBT (Computer Based Training) tutorials, installation guides, technical guides, support contracts, lists of hosting providers, and additional resources. http://techdocs.postgresql.org
Spatially enables the server, allowing it to be used as a backend spatial database for geographic information systems (GIS). http://postgis.refractions.net/
EnterpriseDB is a relational database management system based on PostgreSQL. It adds compatibility and performance features. http://www.enterprisedb.com