Deki Wiki

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.