On Linux I run Pidgin, but on Windows I run Digsby (
www.digsby.com, screenshots on site). It's free to use (there's ads in the conversation windows to pay for development costs, but you can turn them off easily) and I find it much nicer, sleeker and easier to use than any of the alternatives.
Digsby features:
-IM:
AIM, MSN, Yahoo, ICQ, Google Talk, Jabber, and Facebook Chat, plus the ability to send SMS messages
-Email:
Hotmail, Gmail, Yahoo Mail, AOL/AIM Mail, IMAP, and POP
-Social Networking:
Facebook, Twitter, MySpace and LinkedIn, including the ability to browse feeds, get notifications and post statuses.