Kal Ahmed and Christoph Fröhlich have released version 0.9.8 of the TM4J project from sourceforge (https://sourceforge.net/projects/tm4j/).
If you've played with Ontopia's Omnigator and topic maps, the TM4J project is definitely worth trying. The tmnav visual representation of topic maps is really cool.
Steve Pepper gave me a demo of the "Vizigator" at XML 2004 that will be coming out from Ontopia soon, but the tmnav viewer has already been around for a year or so with similar functionality.
See also: TopicMaps