bIRC is a simple, lightweight and flexible IRC-Client written in Bash. bIRC doesn't require an installation and can be launched by executing a single script file. bIRC has plugin support and can be easily modified.
Colloquy Alternatives
Colloquy is described as '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' and is a IRC Client in the social & communications category. There are more than 25 alternatives to Colloquy for a variety of platforms, including Linux, Windows, Mac, Self-Hosted and Web-based apps. The best Colloquy alternative is SeaMonkey, which is both free and Open Source. Other great apps like Colloquy are HexChat, mIRC, irssi and WeeChat.