Well, being a total Apple loyalist, I use iChat, however it only does AIM and Jabber (with Google Talk being a derivative of Jabber). Since I don't expect you'll be as picky about using Apple software only, I can very highly recommend
Adium, which is the de facto standard in instant messaging on the Mac. Supports virtually every protocol known to man and has extensive theming and customization options. The only reason I do not use it is that it is not made by Apple and thus does not offer the same level of integration with other Apple applications (Safari, Mail, Address Book, etc.).