There are lots of open source database available. Here is the list of most popular open source database.
1) MySQL
http://www.mysql.com/
2) PostgreSQL
http://www.postgresql.org/
3) Firebird
http://www.firebirdsql.org/
4) GNU SQL Server
http://panda.ispras.ru/~gsql/
5) ScimoreDB
http://www.scimore.com/
6) DXstore database system
http://www.dss.bc.ca/dx/
7) SAP MaxDB
http://maxdb.sap.com/
8) HSQLDB
http://hsqldb.org/
9) CloudDB
http://www.clouddb.org/CloudDB11/
i hope this are useful for selecting the Open source database.
