Mercury does MSN webcam, but I've found it to run extremely slow.
Look, the reality for me is that IM on Macs simply sucks. The only decent IM on Mac is iChat, but face it, if you don't live in America you won't have a strong affiliation with AOL, and if people don't have Macs then chances are your buddy list is going to be pretty empty.
In Australia almost everyone uses MSN Messenger. MSN Messenger 7.0 for PC is absolutely fabulous (apart from it crashing occasionally but that's to be expected). It has so many more features than other IM programs, it's just a shame that Mac doesn't have it. Especially with so many expansions, Messenger Plus!, etc.
So I don't know what I use. I use iChat to speak to ONE person, and MSN Messenger 4.0.1 to talk to everyone else (which sucks complete and utter ass). I've tried aMSN but it's ugly and unstable so I guess I'm really at a loss. For me the Internet experience on a Mac is great, but IM is simply something that I hate to do on a Mac.
It especially annoys me that Apple provides full featured apps to Windows such as iTunes and QuickTime Player (aside from H.264) and Microsoft offers 10% completed programs to Mac users. But what can you do?