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.
Pidgin supports many features of these chat networks, such as file transfers, away messages, buddy icons, custom smilies, and typing notifications.
Pidgin is free and open source, available under the GNU General Public License.