Firebird

Firebird is an open source relational database management system that runs on Linux, Windows, and a variety of Unix platforms. Firebird offers excellent concurrency, high performance, and powerful language support for stored procedures and triggers.

License: IPL/IDPL
Written in: C++
Operating system: Cross-platform

Website: http://www.firebirdsql.org/
Download: http://www.firebirdsql.org/index.php?op=files
Documentation: http://www.firebirdsql.org/index.php?op=doc