The TMLab is working on a new open source Topic Maps engine in Java called MaJorToM. It implements TMAPI 2.0 and so far has only an in-memory backend. However, this is more than just another Topic Maps engine, since it provides some additional features over and above the usual:
- Special support for modelling time and space via predefined occurrence types,
- Versioning of the topic map,
- Event API (perhaps the least unusual feature),
- Transaction support, even in the in-memory backend.
The engine is not released yet, but the source code is available in the Google Code repository.