Pidgin (formerly named Gaim) is a multi-platform instant messaging client, based on a library named libpurple. Libpurple has support for many commonly used messaging protocols.
Pidgin supports the following Instant Messaging protocols:
Bonjour, Gadu-Gadu, Internet Relay Chat (IRC), Lotus Sametime, MySpaceIM, MXit, .NET Messenger Service ( MSN Messenger or Windows Live Messenger), Novell GroupWise, OSCAR (AIM/ICQ/.Mac), QQ, SIMPLE, SILC, XMPP (Google Talk, LJ Talk, Gizmo5, Facebook Chat, ...), Yahoo, Zephyr.