Pidgin IM

Formerly known as GAIM, Pidgin is an open source Instant Messaging (IM) client. It handles the usual protocols: Bonjour (Apple's implementation of Zeroconf), Gadu-Gadu, Internet Relay Chat, Lotus Sametime, MySpaceIM, MSN Messenger, Novell GroupWise, OSCAR (AIM/ICQ/.Mac), QQ, SIMPLE, SILC, XMPP (Jabber, Google Talk), Yahoo! , Zephyr. You can log in to various accounts on many different networks at the same time, and of course sign in to chat rooms. Thus you do not have to install a client for each network and people can reach you from everywhere. Also you can customize it with numerous plug-ins.