The following are some links to websites regarding IRC clients for Unix and related systems (including Linux).

IRC Clients for X: Large list of clients that can be used on machines with X-windows.

IRC Clients for Unix: Another list from the same site.

Linux IRC Mini-Howto Describes several IRC clients for Linux along with a lot of other educational information.

BitchX A modified clone of the popular ircII client, and is available for almost all UNIX OS's as well as Win9X/NT and OS/2.

Zircon An IRC client for Linux machines running X Windows.