I had three different chats open when my son told me to shutdown iChat, gTalk and Gizmo and download Adium. Why? So I would only need one client to connect to all three services plus a local Jabber server. Those are all I’ve tested so far. He has MSN Messenger on his too so we know that works.
The client, Adium, requires a plug-in for Gizmo or as they call it an Xtra. I added Homestarrunner sounds (also an Xtra) and have turned off the other clients. I added the Aqua Plus Xtra to get the little Gizmo icon. Everything (icons, buddies, etc) moved over seamlessly.
Mac OSX only. Open Source. Community supported.