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AIM has some advertising but it's not horrible.
If you want to go with something clean, go with Trillian:
http://en.wikipedia.org/wiki/Trillia...ging_client%29
It doesn't quite have all the features of AIM, but for basic chat functions it should be fine.
It's all really simple to use.
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You can also try Pidgin (ex GAIM). This is a multi protocl chat client (works with MSN, ICQ, Yahoo etc etc).
Plus : it's an open source program.
Google for Pidgin