WeeChat (Wee Enhanced Environment for Chat) is a free IRC client, fast and light, designed for many operating systems.
Main features are:
multi-servers connection (with SSL, IPv6, proxy)
many GUI: Curses, wxWidgets, Gtk and Qt
small, fast and light
customizable and extensible with plugins and scripts
multi-platform (GNU/Linux, *BSD, MacOS X, Windows and other)
100% GPL, free software
WeeChat homepage is here: http://weechat.flashtux.org