Just before Christmas I was part of a team that launched WikiX an Open Source Wiki for EPiServer CMS 5 R2. Even though it's still in beta, it is actually really nice with a lot of cool features. WikiX is based on some of the thoughts and ideas presented here: http://allantech.blogspot.com/2008/10/wiki-vs-cms-difference-is-psychological.html.
See the announcement: http://labs.episerver.com/en/Blogs/Allan/Dates/112230/12/Announcement-WikiX-is-here/
Try it live here: http://wiki.demo.episerver.com/
Download it from Codeplex:http://www.codeplex.com/EPiServerWiki