Microsoft announced “WikiPlex,” a new re-written CodePlex's wiki engine and released as open source under the MS-PL license. WikiPlex is great if you already have a .NET application you'd like to embed a wiki interface into. Developers that want to leverage CodePlex's wiki engine with their own applications can do so with no problems, since WikiPlex has been made available for download for free. Be it as simple as allowing users to host their own homepage content, item descriptions, or comments - the possibilities are endless!

More info: WikiPlex