There are binaries for many operating systems including Windows. It started out as a script for the ircII client, but then panasync patched the script directly into the actual client in 1994.

BitchX: BitchX is another popular open source text based IRC client for UNIX.

WeeChat: WeeChat is an open source text based client with some advanced features like multiple interface capability, horizontal and vertical window splitting, nickname list, smart 'hotlist' in the status bar to show channel activity by name, highlighting without switching channels, and spell checking.

There are binaries available for several operating systems including Mac OSX and Windows. Irssi also supports Perl scripting and runtime modules, so it is highly customizable. Irssi is different from BitchX because it does not use the old ircII code.

Irssi: Irssi is a popular open source and free text based IRC client.

mIRC is shareware, so to legally run it past 30 days without nag screens you need a $20 license. mIRC is a graphical IRC client for the Windows platform with it's own scripting language.

mIRC (Shareware): Khaled Mardam-Bey released the first version of mIRC on February 28, 1995.

Ircle runs on Mac OS 7.2 through 9.0.x, as well as Mac OS X 10.1 or newer.

Ircle (Shareware): Another IRC client for Mac.

It has several features and is actively developed.

Snak (Shareware): Snak is an IRC client for Macintosh OS X.

Colloquy: Colloquy is a free and open source graphical client with a slick Mac-inspired interface for Mac OS X.

There are versions available for various platforms including Windows, Mac OS X, FreeBSD, Linux, and Solaris.

Opera: The excellent Opera web browser also has a simple built-in IRC client.

Pidgin: Pidgin is an open source, cross-platform application that combines many chat networks (includng IRC) into one program.

There are binaries available for Unix, OS X, and Windows.

KVIrc: KVIrc is another open source and cross-platform IRC client based on the QT toolkit.

Its unique feature is the ability to attach and detach the graphical client portion from the core, similar to running screen and a text based client.

Quassel: Quassel is an open source and cross-platform distributed IRC client.

To see a list of completely free Windows builds, go here). (Note: The 'official' Windows builds are not free. There are builds for various *nix distributions, Mac OS X, and Windows.

XChat: Created by Peter Zelenzy, XChat is a free, open source, and multi-platform IRC client with a graphical user interface.

Free windows irc client