BitchX is a free IRC client. BitchX 1.1 final was released in 2004. It is written in C, and is a console application. A graphical interface is also available, which uses the GTK+ toolkit. It works on most Unix-like operating systems, and is distributed under a BSD license. It is originally based on ircII and eventually it was merged into the EPIC IRC client. It supports IPv6, multiple servers and SSL, but not UTF-8. BitchX (often called just "BX" by fans) is well known for its unique default of sending random messages on a /quit.

Link: http://www.bitchx.org/

ircII is an IRC and ICB client that runs under most UNIX platforms. It comes in source form and is completely free of any charges or fees. The ircII project is a group of people who maintain the source code, incorporating fixes and new features.

Link: http://www.eterna.com.au/ircii/

Irssi is a terminal based IRC client for UNIX systems. It also supports SILC and ICB protocols via plugins.
Irssi will automatically log any channels, queries or special windows that you want.
Theming is an easy yet powerful way to customize your Irssi client's look and feel. Irssi's formatting is modular which means you can just change the appearance of the objects and all items contaning that object will change into that format.

Link: http://irssi.org/