Pidgin is now the new name of GAIM, the popular open source, multi-protocol instant messaging application. I have used Gaim, err Pidgin, before but since I frequently use my Mac, I use AdiumX, which uses the same library. For those not familiar with Pidgin, it supports the following IM protocols:

AIM
Bonjour
Gadu-Gadu
Google Talk
Groupwise
ICQ
IRC
MSN
QQ
SILC
SIMPLE
Sametime
XMPP
Yahoo!
Zephyr

You can download the source and binaries here.

I highly recommend that you try this out, this is the greatest, imho, IM client on Linux.

(image taken from pidgin site)