Good post on wikis and orgs.
Socialtext developer Chris Dent has a long and well considered post on social software and why he does wiki.
I think there are three primary audiences for wikis: the individual who hopes to use the wiki as an outboard brain or memory; the nascent group that hopes to discover and solidify the community that lies as potential in their loose connections; and the existing community that hopes to support a shared goal or perform some action.
(Via Ross Mayfield's Weblog)