XiRCON - Finally, an IRC client for Windows that appeals to users of all skill levels. Instantly usable, endlessly configurable.
XChat - Xchat is a multiplatform IRC Client that works on Windows 95/98/NT/2K/ME/XP, MacOS X, Linux, FreeBSD, OpenBSD, and more.
BitchX - BitchX is a VERY heavily modified ircII client. BitchX is a multiplatform IRC Client that works on Windows 95/98/NT/2K/ME/XP, MacOS X, Linux, FreeBSD, OpenBSD, and more.