Ratchet - A GTK2 client written in Ruby [formerly Rirc]
247 - A SWT client written in Java
Irssi2 - Terminal based client
Yamoxic - Mac OS X IRC client written in Cocoa / Objective C
- Irksu - Has been postponed to the future, sorry
Plix - A platform independent IRC/Icecap client. Written in python with wxPython.
Ricer - Ruby Icecap Robot, an icecap 'bot. ;D
Icegap - Gnome/GTK+ Icecap client