RelationalWebDesign.com

Unity
Another Unix filter implementation of some relational ideas.
www.bell-labs.com/

Jinx
Another relational-like Unix simple files database.
wwwtios.cs.utwente.nl/

NoSQL
A descendant of RDB.
www.scriptaworks.com/

System R
The first implementation corrupted from the relational model, was the basis for SQL and DB2. A site preserving early history of a technology that became a critical business tool.
www.mcjones.org/

rdb
A very simple quasi-relational DBMS based on Unix shell commands.
www.rdb.com/

OpenDB Alliance
An association of database creators and vendors committed to promoting an open source model of a Database structure as an industry-standard for the utilization and exchange of database information and easier production of new applications.
www.opendb.org/

QED
Small footprint database with transactions, failure recovery, SQL 92 conformance plus many SQL3 features. [Commercial]
www.quadcap.com/

Hypersonic SQL
RDBMS with JDBC interface, client-server version, query tool, grid, tree, performance test class, servlet, and self-test program. [Open Source, BSD-like]
hsqldb.sourceforge.net/

Smyle
(Storage That Makes Your Life Easier) An open source hierchical/relational database, seamlessly integrated with Java. [Open Source, GPL]
www.drjava.de/

HyperXtremSQL
A pure Java RDBMS based on HSQLDB (HypersonicSQL). [Commercial]
www.hxsql.com/