Since around the middle of last year, been using Pidgin (formerly "Gaim") mostly as my all-in-one chat thingy. Been quite happy with it. Totally free, open source program, updated regularly. No major problems with that program.
The other day, I saw a couple of folks over on BroadbandReports.com 's forums mention a program called "Digsby". Figured I'd download it and try it out. Not a bad program at all. Still in a "beta" stage, but hasn't crashed once (not YET, anyway). You can put most of your chat client info in it (AIM, ICQ, MSN, Yahoo, Google Talk, Jabber), and social networking sites (Twitter, Facebook, MySpace). The developers aren't charging for the program (not yet, anyway, you never know down the line what might happen with "new" software!). The program has a nice, sleek, skinnable interface, doesn't look quite as "clunky" as Pidgin.