Pidgin is a free multi-protocol Instant Messaging client software that allows you to use all of your IM accounts at once. You can talk to your friends using AIM, ICQ, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, Novell GroupWise Messenger, QQ, Lotus Sametime, SILC, SIMPLE, MySpaceIM, and Zephyr.
Pidgin integrates well with GNOME 2 and KDE 3.1′s system tray, as well as Windows’ own system tray. This allows you to work with Pidgin without requiring the buddy list window to be open at all time

Pidgin was originally known as GTK+ AOL Instant Messenger. In response to pressure from AOL, the program was renamed to the acronymous-but-lowercase Gaim. As AOL’s IM program AOL Instant Messenger gained popularity, AOL trademarked its acronym, “AIM”, leading to a lengthy legal struggle with the program’s creators, who kept the matter largely secret.

Pidgin can log in to multiple accounts on multiple IM networks simultaneously. This means that you can be chatting with friends on AIM, talking to a friend on Yahoo Messenger, and sitting in an IRC channel all at the same time.