I have recently been playing with Adium, which is a multi-protocol IM client for OS X (a bit like gaim, which I like to use on Linux).
Now, don't get me wrong - I love iChat AV, and will definitely be sticking with that on my desktop, where I have a reasonably decent display and an iSight connected, so I can use all the whizzy audio and video features.
However, I spend some of my time surfing on my old PowerBook, where I don't use video (or even audio) that often, and the iChat buddy list takes up a lot of the limited screen real-estate. Adium is just great for this scenario, as the buddy list is customisable, so I have a tiny list of transluscent buddies in the corner of my screen. I'll maybe dump a screen shot up here at some point.
I haven't investigated audio capabilities yet, but for the right usage, it's looking pretty handy so far.