as quoted on the W3C Semantic Web Wiki

These are online search engines specialized in OWL and/or RDF content.
Falcon
Sindice

Sindice is a lookup index for Semantic Web documents. Sindice indexes the Semantic Web and can tell you which sources mention a resource URI, IFP, or keyword. Sindice does not answer triple queries. You can use Sindice in your application to find relevant RDF sources.
Swoogle

Swoogle lets user-agents search for Semantic Web Documents, especially Semantic Web Ontologies.

SWSE

SWSE provides an entity-centric view on Semantic Web instances. SWSE also provides a SPARQL endpoint over currently around 400k RDF files from the Web.