Xirc - Fully featured cocoa IRC client. - http://xoomer.alice.it/gluca.cannata/
MacOSX.com Chat Client - Customized IRC client to connect to the IRC channel created for the MacOSX.com website. - https://sourceforge.net/projects/macosxchat/
Conversation - Client developed with ease of use in mind. Versions for 10.2 and 10.3 available. Documentation, downloads and feature list. Donationware. - http://homepage.mac.com/philrobin/conversation/
MacIrssi - Based on the *nix IRC client irssi. Free, open source. - http://www.g1m0.se/macirssi/
Irssix - A free (GPL) IRC client for Mac OS X based on the UNIX client Irssi. - http://gizmolabs.org/~andrew/andrewweb/project.php?pid=2&tab=0
ChiperSoft Systems - Minerva - Formerly called AthenaIRC. Shareware. Origins and history of the program, feature list with screenshots, downloads, and support forums. - http://www.chipersoft.com/minerva/
X-Chat Aqua - The Mac OS X sister of the Unix program XChat. - http://xchataqua.sourceforge.net/
Snak - Client for MacOS 9 and up. Advanced features including ircII scripting support and parental control. - http://www.snak.com/
iRC - Native Mac OS X IRC client [Shareware]. - http://www.mice-software.com/products.php?product=7
Ircle - IRC client for MacOS. - http://www.ircle.com/
Colloquy - An IRC client for Macintosh OS X. Contains screenshots, documentation, support and download area. - http://colloquy.info/