Pidgin (formerly known as Gaim) is a multi-protocol Instant Messaging client for Windows and Linux that allows you to use all of your Instant Messaging accounts at once.
It can work with:

AIM Bonjour Gadu-Gadu Google Talk Groupwise ICQ

IRC MSN MXit MySpaceIM QQ SILC

SIMPLE Sametime XMPP Yahoo! Zephyr