Adium is a chat client for the Mac that can handle almost any messaging network in existence. While it doesn't provide some features (say, video chat) and I've yet to find ANY Mac AOL client that gracefully deals with AOL chatrooms, this has completely taken over for iChat. The times I'm using iChat it's for screen sharing or video chatting (and Skype has been cutting into that somewhat).
What I Use
There are plenty of articles out there about “must-have apps” and others that evaluate “here are all the programs that do this.” This is not one of them. Instead, this is a tour of the tools I currently use that didn’t come with my Mac, and why. I’m ignoring the built-in apps like the Terminal for command-line access or Disk Utility because if I don’t mention a third-party alternative, I’m currently using it, and you can check it out for yourself. Along the way I’ll mention a few good alternatives that didn’t quite fit my needs.