TM4J - Topic Maps For Java

TM4J is an Open Source Java library for working with TopicMaps. It provides its own API but also provides TMAPI interfaces. TM4J can be used with 3 backends: in-memory, Ozone (an Open Source object-oriented database) and Hibernate (for use with relational databases such as MySQL).

tinyTIM 1.5 - a tiny in memory TMAPI implementation

tinyTIM is an Open Source TMAPI in memory implementation with a small jar footprint. tinyTIM is suitable for Java applets like hypergraph. tinyTIM also implements the latest IndexAPI proposal.

XTM4XMLDB - Topic Maps for XML databases

XTM4XMLDB is an Open Source TMAPI implementation for native XML databases like eXist or Apache Xindice as backend.

OKS - Ontopia Knowledge Suite

The OKS is a commercial suite of Topic Maps tools for building, maintaining, and deploying topic map-based applications. It has a native API, but also supports TMAPI, and adds support for tolog, the Ontopia Schema Language, full-text search, as well as frameworks for building Topic Maps-based web applications.


Source: http://www.tmapi.org