irssi - Irssi is a terminal based IRC client for UNIX systems.
Pidgin - Pidgin is an easy to use and free chat client used by millions. Connect to AIM, MSN, Yahoo, and more chat networks all at once.
KVIrc - KVIrc is a free portable IRC client based on the excellent Qt GUI toolkit.
WeeChat - WeeChat is a fast, light and extensible chat client.
Kiwi IRC - A hand-crafted IRC client that you can enjoy. Designed to be used easily and freely.
Quassel IRC - Quassel IRC is a modern, cross-platform, distributed IRC client based on the Qt4 framework.