Colloquy

If you have IRC conversations on a regular basis, Colloquy is a tool that'll be right up your alley. It's an excellent IRC client that lets you chat over a network made up of servers of this protocol.

It's also compatible with lesser known protocols, like SILC (secure online conference calling in real time) and ICB (a teleconference system based on client-server structure).

Colloquy lets you select the style in which the messages are shown. There are seven styles in total included in the program, with some of them including group messages.

Other interesting features from Colloquy include: emoticons, file transfer, compatibility with AppleScript, extensibility through Plug-Ins, and message coding that uses different character systems.