GAIM is an Open Source 'clone' of America OnLine's Instant Messenger
client using the GTK+ toolkit. It features most of the functionality
of the official AIM client while being much smaller and faster.  It
features multiple groups, group chat, user information fetching and
registration, time stamping, sound (for when you really do want to be
interrupted), multiple away messages, and more.

GAIM also has support for other IM protocols such as Yahoo!, MSN, ICQ,
Jabber, Napster, IRC, and Zephyr via included plugins.

WWW: http://gaim.sourceforge.net/