Colloquy
Colloquy is an open-source IRC, SILC and ICB client for Mac OS X. Colloquy uses its own core, known as Chat Core, although in the past it used irssi as its IRC protocol engine. One of the primary goals behind Colloquy was to create an IRC, SILC and ICB client with Mac OS X visuals. Colloquy contains a user interface that follows Apple's Human interface guidelines in addition to containing support for traditional IRC command-line controls such as /nick and /join.
Features
Colloquy supports a variety of different text modifications. One text manipulation supported by Colloquy is the use of colors as used by mIRC; with the primary colors being: White, Black, Navy, Forest, Red, Maroon, Purple, Orange, Yellow, Green, Teal, Cyan, Blue, Magenta, Grey, and Ash. Additionally, Colloquy supports formatting text with underlining, italics, bold, and outline.
Scripting is another popular feature. Colloquy supports scripting in languages such as AppleScript, F-Script, JavaScript, Objective-C and Python. Colloquy shows changes such as mode changes and ban sets in a human-readable format, rather than showing raw modes.
Plugins and customizability
Colloquy is built on Apple's WebKit engine and supports customizable message views called "styles" using a combination of XSLT, HTML, CSS and JavaScript.
These act like themes, completely altering the way the application displays chat sessions. The software also supports a wide array of plugins that enable customization of the application and integration with other aspects of the Mac OS X environment.