- Apache CouchDB
- NOSQL Relational Database Management System
NoSQL, a GNU GPLed relational database management system, is, as its name suggests, is not an SQL database. It's actually a shell-level tool that creates regular ASCII files that can be manipulated by other text tools (like text editors, etc). It is closely integrated with Unix, and the author created it because he wanted something he could use for simple database management tasks that didn't require the full feature set usually present in other DBMSes.
- LEAP Relational Database Management System (RDBMS)
A relational database management system that comes with full C source code, released under the GNU GPL. According to its website, it is as powerful as the complex offerings from Sybase, Oracle and IBM, but was originally written as an educational tool. It's query language is a full implementation of relational algebra. It supports Windows 95/NT, Linux, Solaris, SunOS, HP/UX, AIX, etc.
- nanoBase 1997
nanoBase is neither a database library nor an SQL server, but a clone of the old dBase. It is an xBase interpreter and it works under MSDOS. It comes with source code, and is apparently released under the GNU GPL.