Gaim - Multi messenger program, so you can be connected to many different chat systems and profile's at once.
Open Source
Kopete - " Kopete is an instant messenger supporting AIM, ICQ, MSN, Yahoo, Jabber, IRC, Novell GroupWise Messenger, Lotus SameTime*, and more. It is designed as a flexible and extensible multi-protocol system using plugins. (* in CVS)"
Open Source
Easy Message - Multi messenger program, so you can be connected to many different chat systems and profile's at once (AOL, MSN, POP3,ICQ, Yahoo).
Freeware
Gaim - Multi messenger program, so you can be connected to many different chat systems and profile's at once.
Open Source
Google Talk - 'Google Talk enables you to call or send instant messages to your friends for free, anytime, anywhere in the world'.
Free
ICQ - "ICQ (or 'I Seek You') instant messenger is used by millions worldwide to make the complicated work of finding friends, colleagues and people with similar interests, as straightforward as calling across a room and starting a friendly conversation." .
Freeware
mIRC - "IRC is a multi-user chat system, where people meet on "channels" (rooms, virtual places, usually with a certain topic of conversation) to talk in groups, or privately."
Shareware
MSN Messenger - Installed by default on many M$ operating systems so it's user base is high but closure of all it's chat rooms may prove to be less useful/fun.
Freeware
Trillian - Multi messenger program, so you can be connected to many different chat systems and profile's at once.
Free version available.
Yahoo Mesenger - Yahoo's messenger program with voice, video conferencing, chat rooms, friends list, pretty good all round concidering the amount of people using the system.
Freeware