Oh come on! No one else is reviewing??
It's free! It's amazing! It's fast, stable, incredibly useful, and easy to learn!!
If I could add just 2 features, they would be (1) more of a WYSIWYG editor, and (2) the ability to assign moderators to pages so that edits are held until approved.
(1) would make this tool much easy to use for non-techies -- I've run into this several times... trying to use a wiki for collaborative document development but having to give up because non-techies were too intimidated or found it too difficult.
(2) Would make it a much more useful tool for collaborative document development and in smaller communities where there is are not enough people in the pool to monitor collectively for mistakes, whackos, etc.
But this is still an awesome tool!