Instant Messaging
Adium X, AOL Instant Messenger, iChat AV, ICQ, JabberFoX, MSN Messenger Service, Nitro, PalaceChat, Proteus, Psi
IRC
AkwaIRC, ChiperSoft Systems - Minerva, Colloquy, Conversation, Crikey, iRC, Ircle, Irssix, MacIrssi, MacOSX.com Chat Client
Charla
Open source client for accessing Yahoo chat networks.
Fire
A multi-protocol IM client for OS X; supports the six most popular instant messaging services as well as IRC chatting.