Universal Instant Messenger: Trillian

I rarely recommend software products but this one is really cool. I dumped my Yahoo messenger and can get instant messages from Yahoo, AOL, etc.

Communicate with Flexibility and Style. Trillian is everything you need for instant messaging. Connect to ICQ®, AOL Instant Messenger(SM), MSN Messenger, Yahoo! Messenger and IRC in a single, sleek and slim interface.