Ontopia is an open source suite of tools for building applications based on Topic Maps, providing features like an ontology designer, an instance data editor, a full-featured query language, web service access points, database storage, and so on.
The product suite is highly mature. Ontopia 1.0 was released in June 2001, and we are now nearing the release of Ontopia 5.1. Ontopia has been in production use in a number of commercial projects on three continents for many years now, and the core engine has been very stable over most of that period.
Ontopia is open source and released under the Apache License 2.0. The entire product is released as open source. There are no proprietary add-ons which are necessary to run it, or to make it suitable for an enterprise setting. Commercial support, however, is available.
Ontopia is being actively maintained and developed by the committers which include two people who have worked on the product for more than a decade, as well as new project participants from Germany, the Netherlands, Belgium, and Norway. The Google Code site has a live feed of recent activity.