Firebird. It meets all of the requirements you've listed.
Firebird is a relational database offering many ANSI SQL standard
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 Project is a commercially independent project of C and C++
programmers, technical advisors and supporters developing and
enhancing a multi-platform relational database management system based
on the source code released by Inprise Corp (now known as Borland
Software Corp) on 25 July, 2000.
FREE LIKE A BIRD. Anyone can build
a custom version of Firebird, as long as the modifications are made
available, under the same IDPL licensing, for others to use and build
FREE LIKE FREE BEER. No fees for download, registration,
licensing or deployment, even you distribute Firebird as part of your
commercial software package.
Firebird's development depends on
voluntary funding by people who benefit from using it. Funding options
range from donations, through Firebird Foundation memberships to