If you're a startup (2 people or less) Fogbugz is free and comes with wiki software. If there's more than two of you, then if PBwiki is expensive..fogbugz is much more so.
Thank you - we want to let the users be allowed to edit the Wiki - hence we're looking for no limit on users. MediaWiki is starting to look more and more attractive but it's just so visually cluttered for my liking.