Jabber is an open protocol which anyone may freely write a client for or even run their own server for, unlike the major IM networks (AIM, Yahoo, etc.). It is not a specific IM program. There is a page listing all sorts of programs with Jabber functionality.
http://www.jabber.org/software/clients.php
Gaim is a multi-server IM client like Trillian, and is totally free. I forget whether it supports Jabber by default or whether you need a plugin for Jabber though...