If you are thinking about building applications based on topic maps and do not feel like shelling out money for proprietary software, then do not look any further than Ontopia! Ontopia is an open source tools suite with features such as an ontology designer, a full-featured query language; web services points, database storage, and an instance data editor. There are many more powerful tools available with Ontopia outlined here.
Ontopia has been an on-going project in the open source community for over a decade and has an interesting history:
“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.”
A developer community that has been attached to the project for years keeps up Ontopia and there are new participants from Europe. If you are curious about recent activity with Ontopia, they keep a page with Google Code and they also recently updated the Web site’s design.