Firebird is a relational database offering many ANSI SQL-99 features
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.  It has been used in
production systems, under a variety of names since 1981.

Firebird is completely free of any registration, licensing or deployment
fees.  It may be deployed freely for use with any third-party software,
whether commercial or not.

See also: https://sourceforge.net/projects/firebird/