Pidgin is a chat program which lets you log in to accounts on multiple chat networks simultaneously.

Pidgin runs on Windows, Linux, and other UNIX operating systems.

Pidgin is compatible with the following chat networks out of the box: AIM, ICQ, Google Talk, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, Novell GroupWise Messenger, QQ, Lotus Sametime, SILC, SIMPLE, MySpaceIM, and Zephyr. It can support many more with plugins.
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