ProofWiki

Description
"'ProofWiki is an online compendium of mathematical proofs. Our goal is the collection, collaboration and classification of mathematical proofs. If you are interested in helping create an online resource for  proofs feel free to create an account and contribute. Thanks and enjoy!'"

(As of: 09 January 2012)

Related wiki
Many  wiki; also
 * LiteratePrograms
 * Proof General Wiki
 * Wikibooks: Mathematical Proof
 * Wikibooks: Famous Theorems of Mathematics