You are hereEngine / TM4J
TM4J was the leading open source Topic map engine for quite some time, however, recently it has been surpassed by the Ontopia Project going open source. It has also lagged behind in implementation of TMAPI2.0 as the project lead, Kal Ahmed, has focussed on working at NetworkedPlanet. Nonetheless, quite a few projects are based on it.
TM4J supports both in memory and database stores as well as TMAPI. The project also has a home at Sourceforge.