IRC Clients:

Amiga
amIRC http://www.vapor.com/products/?prod=amirc
Chatbox http://www.hut.fi/~samzait/chatbox/

BeOS
Baxter http://www.abisoft.com/Baxter/
Bowser http://bowser.sourceforge.net
LnIRC http://www.braprogram.com/LnIRC/

Macintosh
IRCle http://ircle.netmug.org
MacIRC http://www.macirc.com
ShadowIRC http://www.shadowirc.com
Snak http://www.snak.com/Snak.html
Palm Pilot
upIRC http://www.smittyware.com/palm/upirc/

Unix
BitchX http://www.bitchx.org
EPIC http://www.epicsol.org
ircII http://www.eterna.com.au/ircii/
irssi http://irssi.org/
kvirc http://www.kvirc.net
ScrollZ http://www.scrollz.com/
sIRC http://www.iagora.com/~espel/sirc/sirc.html
X-chat http://www.xchat.org
Zircon http://catless.ncl.ac.uk/Programs/Zircon/

Windows
dIRC http://www.dragonmount.net/software/dirc/
Klient http://www.klient.com
Leafchat http://www.leafdigital.com/Software/leafChat/
mIRC http://www.mirc.com
WiRC http://wircirc.cjb.net
XiRCON http://www.xircon.com

WindowsCE
IrcCE http://www.movsoftware.com/products/ircce/ircce.htm
PocketChat http://www.sspocketchat.com/