Shtuff Chat - Free Group / Team chat with persistent history.
HexChat - HexChat is a fork of XChat with bug fixes and new features.
JabbR - A chat application built with ASP.NET and SignalR.
irssi - Irssi is a terminal based IRC client for UNIX systems.
Smarsh Instant Messenger - smarshIM Mobile is a free application download that requires a paid account on the smarshIM service...
Adium - Adium is a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.