The Firebird Project's Firebird Relational Database
Firebird distills its identity into the phrase "True universal open-source database" and boasts not only of being "free like free beer" but also, fittingly, of being "free like a bird". The latter permits anyone to build a custom version of the Firebird, as long as the modifications are made available for others to use and build upon.
Technically speaking, Firebird is a relational database offering many ANSI SQL standard features and runs on Linux, Windows and various UNIX platforms. Firebird 3.0 is the latest major release that succeeds most notably in unifying the server architecture and improving support for SMP and multiple-core hardware platforms. Parallel gains in 3.0 include an improvement in the threading of engine processes and the options for sharing page cache across thread and connection boundaries, among other improvements.