I just checked out pbwiki.com
Its a cool site allowing you to create a free wiki which you can set up to be modified by anyone. They are pretty cool for a free service and the upgrades available are cool too. They let you attach a domain to the wiki and modify template more easily.
I’m working on a 3D Galaxy wiki. Feel free to make any changes to it you want because its still brand new and in development.
I’m thinking of putting a help section in there among some other stuff but go ahead and ad anything you want.
The other well known wiki site is media wiki, the engine used to power the huge online encyclopedia wikipedia. The only drawback of getting a mediawiki is you need to have webspace with no ads, a mysql database and php to be able to use it, but if you do I would probably reccomend it.