Miranda is a popular choice for Windows systems:
Miranda IM is a multi protocol instant messenger client for Windows. Miranda IM uses very little memory and is extremely fast. It requires no installation and can be fitted on a single floppy disc. Its powerful plugin system makes Miranda IM very flexible. Only the most basic features are built in, but there are currently more than 350 free plugins available for download that allows users to extend the functionality of Miranda IM.
Adium is the IM application of choice under Mac OD X:
Adium is a free instant messaging application for Mac OS X, released under the GNU GPL and developed by the Adium team. With Adium, you can connect to any number of messaging accounts on any combination of supported messaging services and then chat with other people using those services.
Meebo is quickly gaining support as the IM client for mobile devices:
meebo.com is a website for instant messaging from absolutely anywhere. Whether you’re at home, on campus, at work, or traveling foreign lands, hop over to meebo.com on any computer to access all of your buddies (on AIM, Yahoo!, MSN, Google Talk, ICQ and Jabber) and chat with them, no downloads or installs required, for free!