Adium X

Adium X is an Instant Messaging (IM) application that runs on the Mac OS X. Adium supports AIM, ICQ, MSN, Yahoo!, Rendezvous, Yahoo! Japan, Gadu-Gadu, Jabber, Napster, Lotus Sametime (via the Meanwhile plugin), and Novell GroupWise. It is freeware. It also supports tabbed chatting, and also file transfer via some networks.

It can be customised by downloading scripts, addons, themes and more from adiumxtras.com.