Pidgin

Pidgin: Universal instant messenger client

Pidgin is a free multi-protocol IM client that supports AIM, Yahoo, MSN Messenger, ICQ, Gadu-Gadu, IRC, Jabber, Zephyr and many other networks.
With Pidgin you can login and use all your IM accounts straight from its interface so you can chat with contacts from any of them simultaneously and without having to install dozens of instant messaging applications.

Also Pidgin supports many features of these chat networks, such as file transfers, away messages, custom smileys, buddy icons and typing notifications.
Pidgin has a flexible architecture and its functionality as well as the number of supported networks can be extended through plugins.