XChat

Xchat is a popular free IRC client for Unix and Unix-like operating systems, like Linux, with a version for Windows. Its graphical user interface uses the GTK+ graphics toolkit, featuring the use of tabbed windows to represent IRC server connections, channels and private messages.

XChat is highly extensible, featuring a plug-in architecture able to accept Perl, Python and binary (usually C) modules. It is released under the GNU General Public License.

Homepage: http://www.xchat.org