Yesterday, I had to add Yahoo IM client to communicate with team members of a project I've just joined.  With three IM clients running (MSN, AIM, and Yahoo), I had three contact lists and UIs to get confused with.  Yikes.  Then I remembered Trillian which supported all three IM protocols.  It took me about 5 minutes to replace the three IM clients with Trillian.  So far, I've had no problem at all and am happy with it.