The Pidgin chat client is compatible with AIM, ICQ, Google Talk, Jabber/XMPP, MSN Messenger, Yahoo!, Bonjour, Gadu-Gadu, IRC, Lotus Sametime ... everything. It can run on Windows, Linux, and Unix natively. (For OS X, Pidgin recommends sister application Adium.) Pidgin supports buddy lists, file transfers, emoticons, away messages, and nearly 100 languages.
Lots of plug-ins are available as well -- for Facebook, Windows 7 Taskbar integration, text to speech, and audible alerts to name a few. Pidgin's voice and video support is limited to XMPP (Extensible Messaging and Presence Protocol) networks, so most people use it only for text chat and file transfer. Its biggest advantage is that it supports so many different proprietary chat networks all in the same application.
-- Joseph Roth