Showing software for all platforms
Adium is a free instant messaging application for Mac OS X. It can connect to AIM, ICQ, MobileMe, MSN Messenger, Jabber, Google Talk, LiveJournal, Yahoo! Messenger, Bonjour, MySpace IM, Facebook Chat, Lotus Sametime, Novell Groupwise, QQ, Gadu-Gadu, and more.
Miranda IM is a multiprotocol instant messaging client for Windows. Very light on system resources and extremely fast, Miranda IM requires no installation and can be made to fit on a single diskette or USB drive. It features a powerful plugin-based framework and more than 350 available plugins.
Psi is a free, lightweight instant messaging application designed for the Jabber IM network (including Google Talk). It offers Unicode support and localizations, easy file transfers, and customizable icon sets.