Another new tool found via Benoit Brosseau is Deki Wiki. This product from MindTouch is open source and seems to have all the right attributes to make it wildly popular:
Similar to CMS web frameworks like Drupal, Mambo, Joomla and DotNetNuke, Deki Wiki delivers a remarkably extensible platform, but it’s a wiki in nature; therefore making it community-centric and significantly easier for end-users to participate. Also, it has a complete application programming interface (API) for programmers.
Deki Wiki is available as a free download; a free hosted service; or with enterprise-level support.
