Using http://www.google.com/search?q=filetype:owl+owl, you can Google for ontologies.

Currently, Google returns over 53,000 results.

If you prefer, you can Swoogle:

Swoogle (http://swoogle.umbc.edu/) is an specialized web search engine that discovers, analyzes and indexes knowledge encoded in semantic web documents published on the Web. Swoogle reasons about these documents and their constituent parts (e.g., terms, individuals, triples) and records meaningful metadata about them. Swoogle provides webscale semantic web data access service, which helps human users and software systems to find relevant documents, terms and triples, via its search and navigation services. Swoogle also provides a customizable algorithm inspired by Google’s PageRank algorithm but adapted to the semantics and use patterns found in semantic web documents. Swoogle currently has indexed nearly 1.3M Semantic Web documents which contain almost 240M triples. In addition to providing general Semantic Web search services, Swoogle has been used by several projects to maintain and manage specialized collections of RDF data.

Currently, Swoogle searches over 10,000 ontologies – even more if you register.

Swoogle’s stats provide even more meaningful and impressive numbers:

• Swoogle’s aware of about 1.2 million error-free Semantic Web documents
• Swoogle was able to parse over 412 million triples from Semantic Web documents

Based on these data points, ontology developers would be wise to consult Google or Swoogle to avoid reinventing that which already exists … !

When using pre-existing ontologies, it’s also important to remember that as developers you have various choices:

• You can import parts or whole ontologies
• You can integrate ontologies – a pressing need I blogged about elsewhere

Existing ontologies are inherently reusable, extensible and refactorable.