Ontopia - the product
Ontopia is a set of tools which contains everything you need to build a full Topic Maps-based application. Using Ontopia you can design your ontology, populate the topic map manually and/or automatically, build the user interface, show graphical visualizations of the topic map, and much more.
The core of Ontopia is the engine, which stores and maintains the topic maps, and has an extensive Java API. On top of it are built a number of additional components, as shown in the diagram below. More information about these components can be found on the right.
Ontopia is 100% Java, and runs on any operating system which has Java 1.5. It is fully open source and can be used without any restrictions beyond those in the Apache 2.0 license.