X-Chat 2 Windows version, which uses GTK+ Runtime Environment. Compiled and distributed by Vaidrius 'SilvereX' Petrauskas.

savIRC is an Open Source IRC Client written in Tcl/Tk.

HydraIRC is an open-source IRC client with an attractive and easy to use interface. It supports DCC Chat and File transfers, Connecting to multiple servers, Dockable Floating Tabbed Windows, Autohiding Windows, DLL Plugins, Channel Monitoring, Message Logs, Event Viewer, Themes, Color schemes, Buddy groups, Audible and Visual Notifications , Reg-Exp Highlighting, Favorites and much more.

Bersirc is a multi-platform open source IRC client currently in development for the Windows, Linux, and Mac OS X operating systems. It utilises the Claro toolkit, which aims to provide an interface to native windowing systems and widgets on all operating systems. Microsoft .NET and Qt toolkit ports are also planned.

ChatZilla is a cross platform messaging client that combines Internet Relay Chat (IRC) with existing web standards like JavaScript, HTML, and CSS. If you don't know any of this stuff don't worry, you don't have to. ChatZilla is easy to use.