Kiwi IRC - A hand-crafted IRC client that you can enjoy. Designed to be used easily and freely.
mIRC - mIRC: Internet Relay Chat client
Colloquy - Traditionally, chat clients on the Mac have been anything but glamorous.
HexChat - HexChat is a fork of XChat with bug fixes and new features.
Terminal on FB Messenger - Take control of your terminal through Facebook Messenger
irssi - Irssi is a terminal based IRC client for UNIX systems.