Pidgin IM
A universal instant messaging (IM) program
...It runs on Windows, Linux, and other UNIX operating systems. Pidgin is compatible with the following chat networks out of the box: AIM, ICQ, Google Talk, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, MXit, Novell GroupWise Messenger, Lotus Sametime, SILC, SIMPLE, MySpaceIM, and Zephyr. It is written in C and makes heavy use of GLib and GTK+.
Finch is a command line instant messaging program. It also lets you log in to accounts on multiple chat network simultaneously, and it is compatible with the same chat networks as Pidgin. ...