adium is teh koool.
(note: preview's jpg compression is not teh koool).
adium has a bazillion features and preferences, but they're not all in front of you. the 'hidden' preferences are under the advanced tab. neat.
you can receive file transfers, but not send them. they're working on it. it's got full support for buddy icons and the such. remember, adium 2 is an ALPHA. i believe the latest version crashes when you try and edit buddies.
the developers are really cool guys. check out
www.adiumx.com. i've been following the project for a good long while (oh, say, ever since the first binaries).
it's got an itunes plugin for parsing away messages, idle messages or profiles ("i am listening to %_track by %_artist"). it's got an event bezel, fully customizable for login/outs/idles/aways/messages.
the tabbed interface is amazing. dragging tabs around is so much fun. the tabs even have typing/idle/away notifications. when a user goes away, his away message shows up in your chat window. the contact list mouse-overs must be seen to be believed. it's got contact alerts that are fully customizable to a crazy degree. it's also got address book integration (import and export).
it supports msn, jabber, aim, icq, and some other stuff (sorta. they're still working on it. alpha-- remember?)
it's really a community effort. if you have an idea, a UI change, etc. the developers are really responsive.
i promise i'm not working on adium myself. i'm not even a member of their messageboard, but i believe the adium project is an amazing project that is coming along great, that's why i'm sticking up for it. i used ichat for awhile, even while following adium, but as of november or so, i switched over to adium. neato.