No criteria, but I believe google academics is not only simple but use good data mining techniques in the search of patterns and ontologies. Of course we can compare the google results with other search machines in order to form patterns and find divergences! Regards, Farley.

Swoogle : http://swoogle.umbc.edu/

 Schemapedia: http://schemapedia.com

You can find some useful ontologies in Protege Ontology Library also.

Swoogle : http://swoogle.umbc.edu/

Do you know SWEET ontologies? https://sweet.jpl.nasa.gov/

I have used the following list of semantic search engines for Ontologies http://sindice.com/

http://ws.nju.edu.cn/falcons

http://ws.nju.edu.cn/ontosearch

http://swoogle.umbc.edu/

http://swse.org/

http://watson.kmi.open.ac.uk/WatsonWUI/

http://www.semanticwebsearch.com/query/

http://schemapedia.com/

http://dipper.csd.abdn.ac.uk/OntoSearch/SearchEngine

Swoogle is ok! 

http://swoogle.umbc.edu/

I was going to say, don't forget Linked Open Vocabularies, but site is not working today.  http://labs.mondeca.com/dataset/lov/index.html  .  Anyone know what's up?

Swoogle seems to be offline also.

Thank you all so much!!! I made a list with the provided links and I'm testing which are still active and updated.

Here's the actual link to Linked Open Vocabularies:  http://lov.okfn.org/dataset/lov

according to my experience and research the best one are follows

http://swoogle.umbc.edu/

http://watson.kmi.open.ac.uk/WatsonWUI/

Thank you all! 

Laura 

Have you try https://duckduckgo.com. Some of the search engine above don't work

I found the following partially useful:

• https://www.ebi.ac.uk/ols/index (EMBL-EBI Ontology Lookup Service, biomedical ontologies only)
• https://github.com/search?q=ontology
• https://ontohub.org/

However, in my opinion it is rather unsatisfying that semantic web technology, which aims to simplify the access to information, is itself documented and made available in such a scattered and inconsistent way like it is today: many outdated or unreachable sites (see answers above, and also last part of this blog post: https://cknoll.github.io/semantic-python-subjective-notes-en.html ), little tool support (apart from protoge and owl-api).

From my perspective a desirable situation would be like for "numerical machine learning" (neural networks etc.): lots of up-to-date documentation and tools (in various languages).