Mac OS X: Textual is a streamlined open-source IRC client for Mac OS X with an emphasis on a clean interface and easy scripting.
Textual, an IRC client from the developers of ScreenGrab, is based off the open-source IRC client for Mac OS X Limechat. If you're looking for a sleek client with a minimal interface but the ability to download and create your own scripts, Textual offers chat highlighting, shortcuts, scripts, and more. For IRC users familiar with popular Windows client mIRC, Textual feels much like a a sleek version of mIRC that got the full minimalist Mac treatment.
Textual is open-source and Mac OS X only.