From Compsci.ca Wiki
X-Chat is a popular IRC Client for unix based systems, though it has also been ported to Windows. It offers support for connecting to multiple servers, and can be scripted with plug-ins written in Perl, Python, Tcl, and Ruby. It is quite lightweight, compared to other popular IRC Clients such as mIRC and HydraIRC.
X-Chat is free for Linux, BSD, Unix, and Mac OS X. Officially, the Windows version of X-Chat is shareware only, but an unofficial (but fully featured and no different from the official version) build is available. See below for download information.
Official Website - Official, free downloads for most OS's