The definitive listing of wiki forums generally open to the public can now be found at the Wiki Index project.
See also:
Category Wiki Forum lists pages on Wiki Wiki Web about other wikis.
Items tagged with "wiki" on Del Icio Us: del.icio.us 
Another fun way of finding Wiki Forums is to do a whois search for "wiki":
For general information about starting your own wiki, source code, etc., see Wiki Wiki Clone, Wiki Wiki Clones Questions.
See original on c2.com