CouchDB is a peer-based distributed schema-free database system that uses JavaScript as its query language and stores data as independent JSON documents. Access to the database is via HTTP. It is open source and works on Windows, Mac OS X, Linux, Solaris, FreeBSD, OpenBSD, NetBSD, Android and possibly other platforms.

MongoDB is an open source document-oriented database (using JSON style documents with dynamic schemas). Your databases can be indexed on any attribute and files in the database can be of any size. The software supports replication, so that you can mirror your database. The database is scalable through an auto-sharding architecture. There is built-in support for automatic balancing, automatic failover, no single points of failure, etc. Other features include atomic, in-place updating, map/reduce for batch processing and data aggregation, etc. The software can be used on both 64 bit and 32 bit versions of Windows, Linux, Mac OS X and Solaris, although the 32 bit versions are limited to 2 GB of data. The database server itself is written in C++. Your client program using the database, however, can use any language that is supported by the drivers. At this time, this includesC, C++, Erlang,C#, Haskell,JavaScript,Java,Perl,PHP, Ruby, Python, and Scala. There is also third party support for many other languages.

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.

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 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.