We use Fogbugz for the wikis, the discussion groups and focused discussion of particular cases. For instant messaging, we use Sametime. This combination has turned out to be very powerful for us because it provides piles of functionality without forcing a lot of interface onto us. Low ceremony is good.
NOTE - instant messaging is the only aspect of Sametime that we use - I guess there are a lot of other crazy things that you can do and we are completely uninterested in.
