This weekend, I downloaded the latest version of Proteus, and I really like it. It now integrates well with Apple's Address book and displays the full name for a user, instead of the IM username. It also displays the picture that is assigned to the card in Address Book.
The other thing that proteus does well is integrate multiple IM protocols. It works with AIM, MSN, Yahoo, ICQ, Jabber, iChat Rendezvous (or Bonjour), Gadu Gadu, and Sametime. While Apple's iChat only supports AIM, iChat Rendezvous, and Jabber (in Mac OS 10.4)
Proteus does not support the AV features of iChat though.