IRC Clients
Windows
mIRC32/mIRC16/mIRCFAQ	mIRC is the most commonly used client for Windows. It is easy to setup with a good user interface.
Pirch32/Pirch16	The popular alterative to mIRC.
Virc	Virc is a much more powerful than Pirch or mIRC especially for scripting. It might be a little hard to set up for people new to IRC.
Mac OS
Ircle	The most commonly used Mac client.
Unix
ircII	The most commonly used Unix based IRC client.
Acorn
iChat	Popular client for Acorn systems
Torque	Popular client for Acorn systems
Popups
Wolfserv
mIRC	mIRC popups for WolfServ
pIRCh	pIRCh popups for WolfServ
vIRC	vIRC popups for WolfServ (Still needed)
Miscellanea
J-Skrypt v0.4e	Jam's ViRC script
HotPops	A collection of hugs, slaps and other fun popups for mIRC
FunPops	Hugs, slaps, pranks and other fun popups for mIRC
ASCII Art	A collection of ASCII ART popups for mIRC (the funpops has a built in ASCII player)
Gibber	A cool encryption script wrote by StevenRoy

For more information, go to What is IRC?