From W3C Wiki
Semantic Web search engines are applications for finding ontologies that require reasonable effort: queries are usually written as natural language keywords and results are ranked. Some additional information is often provided.
The problem here is that it is still hard to choose between two matching ontologies. What should guide publishers to the right choice? Should these ontologies be reused at all? See also BuildOrBuyTerms.
Description: generic Semantic Web document search.
Available at: http://sindice.com/
Unique strength: indexes dozens of million of semantic web documents which are constantly updated. Updates are managed efficiently such that data are "quasi synchronised" with the live data. It has a powerful mashup interface called Sig.ma.
Description: has a term search feature as well as an ontology search feature.
Unique strength: offers entity search, ontological term search or ontology search services all in a unified interface.
Description: it is the grand-father of Semantic Web search engines.
Available at: http://swoogle.umbc.edu/
Unique strength: first.
Description: SWSE stands for Semantic Web Search Engine. It is an RDF entity search engine.
Available at: http://swse.org/
Description: it is an ontology search engine, that is, it does not try to index all semantic web documents but specifically focuses on Web ontologies.
Available at: http://watson.kmi.open.ac.uk/WatsonWUI/
Unique strength: ontology specific, provides some additional metadata about the ontologies, such as the expressiveness of the axioms and possibly some reviews.
Description: it is yet another search engine.
Available at: http://www.semanticwebsearch.com/query/