Pidgin is a multi-protocol instant messaging program that works with Windows, Linux, and Unix based systems. When I say it is a multi-protocol messenger, I mean that it works with:

AIM
Bonjour
Gadu-Gadu
Google Talk
Groupwise
ICQ

IRC
MSN
MySpaceIM
QQ
SILC

SIMPLE
Sametime
XMPP
Yahoo!
Zephyr

With pidgin you can be logged into multiple accounts at the same time. You can be instant messaging your friend on AIM and another friend on Yahoo! at the same time. Pidgin supports things like file transfer and away messages. With the buddy pounce feature you can have it set to play a sound, start a program, or even send a message when a friend comes online, signs off, or comes away from being idle.