BitchX is a free IRC client. BitchX 1.1 final was released in 2004. It is written in C, and is a console application. A graphical interface is also available, which uses the GTK+ toolkit. It works on most Unix-like operating systems, and is distributed under a BSD license. It is originally based on ircII and eventually it was merged into the EPIC IRC client. It supports IPv6, multiple servers and SSL, but not UTF-8. BitchX (often called just "BX" by fans) is well known for its unique default of sending random messages on a /quit.

Link: http://www.bitchx.org/

Traditionally, chat clients on the Mac have been anything but glamorous. Colloquy is an advanced IRC, SILC & ICB client which aims to fill this void. By adhering to Mac OS X interface conventions, Colloquy has the look and feel of a quality Mac application.

Link: http://colloquy.info/

ircII is an IRC and ICB client that runs under most UNIX platforms. It comes in source form and is completely free of any charges or fees. The ircII project is a group of people who maintain the source code, incorporating fixes and new features.

Link: http://www.eterna.com.au/ircii/

Snak is a modern IRC client for Macintosh. Snak supports an unlimited number of connections and channels, private chats, as well as full Direct Client-to-Client support for filetransfers and chat. It can be scripted with AppleScript and the ircII scripting language.
Snak has several features that are not found in other IRC clients including the ability for multiple panels to share the windows. A panel can contain a channel, a dialog with another user, a list, or information about a DCC filetransfer. This results in an effective use of the screen space and improves the ability to follow multiple channels. A panel can be moved from one window to another by dragging the title bar or the panel tab.
Another feature unique to Snak is the Actions list which makes it easy to automate responses to many common events on IRC.
Snak is distributed as shareware, which means that it can be freely used and evaluated for 30 days at no charge. After the 30-day evaluation period has ended, the program will quit after 15 minutes of use. To remove this limit, users need to obtain a registration key at a price of $29 for a single user.

Link: http://www.snak.com/