The 800 pound gorilla of the wiki world :-); famous through its use by Wikipedia. As a result it has a vast developer community, and excellent language support among numerous other great features.
wikia.com is a web service running MediaWiki.
See http://www.mediawiki.org/wiki/Semantic_Bundle and http://semantic-mediawiki.org/wiki/Semantic_MediaWiki for semantic stuff in MediaWiki.