Fremantle
· Cargo · Semantic MediaWiki · MediaWiki · Wikibase ·
There's a new page on MediaWiki.org called Managing data in MediaWiki. It compares the Wikibase, Semantic MediaWiki, and Cargo extensions. All three of these extensions are about managing the metadata of wiki pages or of concepts represented by those pages.
The thing I love about MediaWiki is that one can give in to the impulse to progressively, gradually, move towards being exact in the representation of data. Each of the above extensions help massively with this, but it's still pretty easy to get confused about how to do it.