Where can I find useful ontologies?

Often stated questions are "Where can I find an ontology for XY?"

Much more helpful, I think, would be the answer to the question, how to find useful ontologies in general. So, what directories or search engines do you know to find ontologies? Or what are your ideas to make finding ontologies easier?

Just one example I know (I'll make a list from your answers):

Ok, here is the list so far (Thanks to all contributors):

I've used:

Plus asking on lists especially the W3C lists - semantic-web and public-lod.

The Ontology design patterns site can also be useful.

Swoogle, which is still being maintained, was originally created to find and rank ontologies as opposed to a-box data. Contact us if you are interested in getting an API key or have needs that the API does not satisfy.


The TONES ontology repository maintained by Manchester. But you could make your question a lot easier to answer if you were more specific about what you mean by "useful". And by the way, Google is also quite useful for finding ontologies - do a search for a keyword plus filetype:owl (or rdf or obo - whatever floats your boat).

Finally see semantic search engines such as SWOOGLE etc...

Another question was asked on the same topic at http://www.semanticoverflow.com/questions/1456/struggling-with-ontologies-where-do-i-begin-and-what-do-i-need-to-know. Some complementary answers have been given.