I would also like to mention that I’ve seen like 5 wikis in the past 24 hours, on all kinds of projects (like Parrot development and FOAF, below). I even saw twiki (the one I’ve set up at work) being used.
TWiki is particularly good because it’s backed by RCS, allows file attachments, has a lot of optional stuff, and uses flatfiles (so it would theoretically be easy to move the data elsewhere). The bad side is that it does use flatfiles, so stuff gets slow and it has to dynamically create every page, often via CGI (i.e. starting a program for every page hit.
update: just found another link, to the Wikipedia’s current events