X-Chat is a popular open-source IRC client available for Windows, Mac OS X and Unix-like systems programmed by Peter Železný. X-Chat uses GTK+ to draw its interface, and so is easily ported to other operating systems. The GUI of X-Chat is intended to both feel and operate in a similar manner as to that of mIRC on Windows, and so implements a tabbed interface to switch between channels as well as a userlist available on either side of the main chat window.
X-Chat aims to be a feature complete client rather than one that is especially suited to new users. It includes support for DCC, CTCP, mIRC colours, op actions (kick, ban, room topic, etc.), aliases, popups, and server lists. X-Chat has a rich plug-in architecture allowing plug-ins to be written in many languages, the most popular of which being Perl, Python and TCL.
There is a fork of X-Chat lead by Steve Green to create a native OS X version called X-Chat Aqua which uses Aqua instead of GTK for the GUI. This is preferred by most OS X users as it integrates much more smoothly with the look and feel of the rest of the OS.
While X-Chat is open-source software, officially compiled binaries for windows are not free, they are instead distributed as shareware. However, as anyone is able to compile from source there are third party binaries available free of charge.
X-Chat can be downloaded from http://www.xchat.org.
X-Chat Aqua can be downloaded from http://www.macupdate.com/info.php/id/11070 | 
