Lists of ontologies
There are several webpages that reference ontologies by simply matching a theme (e.g., People, Product) to a URI.
This category requires minimal effort: if the publisher's data are in the domains referenced in the list, the corresponding ontology can readily used.
These lists pose the question "how to define what's in these lists?" Popularity is one aspects, quality may be another. What is a quality ontology? When does it become popular? Who decides?
Description: this website gives a short list of ontologies, classified by themes (People + Organisations, Places, Events, Social Media, Topics + Tags, eCommerce, More...).
Available at: http://semanticweb.org/, bottom right.
Unique strength: concise, straight to the point.
Possible drawback: too simple, too small. Is it maintained? Arbitrary?
Description: it provides links to ontologies, answering simple questions (how about music collections?);
Available at: http://esw.w3.org/VocabularyMarket
Possible drawback: needs cleaning, some old links not valid anymore.
Description: Semanticweb.org has a list of ontologies, ranked according to their usage.
Available at: http://semanticweb.org/wiki/Ontology
Description: A list of ontologies that are fully documented, used by independent data publishers and possibly supported by existing tools.
Available at: Good Ontologies on ESW wiki.
Unique strength: There are well defined criteria for being a member of this list.
Possible drawback: The criteria do not guarantee that the ontologies are good.