Pidgin Internet Messenger

Pidgin is a chat program that replaces AIM, MSN, Google Talk, Yahoo! and other instant messaging applications. You can log into your AIM, MSN, and Google Talk accounts and chat with people on different networks simultaneously.

Pidgin supports many features of these chat networks, such as file transfers, away messages, buddy icons, custom smilies, and typing notifications. Numerous plugins also extend Pidgin's functionality above and beyond the standard features.
Similar applications: 
AOL Instant Messenger (AIM), MS Windows Live, MSN Messenger, ICQ, Yahoo! Messenger, MySpace IM, Adium, Gaim, Trillian