I would like to establish a list of ontologies/vocabularies that match the following criteria:
they are fully documented, that is, there exists a documentation online that describe the ontology itself and all the terms it defines (provide a link to the documentation);
they are used by at least 2 independent datasets (please provide a derefencable link to the datasets as a proof);
they are supported by tools (give examples or a link to the tools or to a list of tools);
additionally, you can provide a link to a project webpage;
you can also give technical details about the ontology, such as the number of classes it define, etc.
Please, I would like you to provide separate answers for each ontology/vocabulary you mention so that people can vote individually for an ontology.
Edit: I put here for convenience direct links to the ontologies given in the answers, in order of appearance.
Technicalities: Lightweight RDFS vocabulary containing 3 classes and 13 properties. voiD vocabulary directly re-uses terms from scovo: and dctypes: namespaces, and recommends use alongside FOAF and DC(Terms) for describing datasets.
Description: this ontology is used to describe online communities such as forums, blogs, mailing lists, wikis. It complements FOAF by stressing on the description of the products of those communities (posts, replies, threads, etc).
Which datasets use it: SIOC is used on many different websites. Notable examples are: identi.ca (http://identi.ca/danbri/foaf) or CC car pictures (http://carpictures.cc/cars/photo/index.rdf). See also a list of (mostly broken) links to sites using SIOC.
Technicalities: SIOC is a lightweight ontology (17 classes, 61 object properties, 25 datatype properties). It is essentially in OWL 2 DL, with tiny syntactical issues that strictly speaking makes it a non DL ontology. It only relies on OWL 2 RL constructs.
There exists other "subvocabularies" for SIOC with domain names http://rdfs.org/sioc/xxxx# but I am not aware whether these ones are used or supported.
Which datasets use it: Library of Congress and New York Times subject headers, United Nations Food and Agriculture Organization's AGROVOC thesaurus, NASA, IPTC, lots more. See http://www.w3.org/2001/sw/wiki/SKOS/Datasets.