Dataman is a fully multi-threaded database server and client API libraries in C, C++, Java, and a PHP module has now been added as well. This is a 'direct access' database, not a relational one. It emulates the verbs of the EDITOR language.
zTable is a mini-DBMS written in Java. It has 2 parts: a back-end DBMS engine and a front-end Eclipse editor plugin. The engine can be embeded in any Java application. The Eclipse editor can be useful as a free DBMS for end-users.