Psi is a free instant messaging application designed for the Jabber IM network (including Google Talk). Fast and lightweight, Psi is fully open-source and compatible with Windows, Linux, and Mac OS X.
With Psi's full Unicode support and localizations, easy file transfers, customizable iconsets, and many other great features, you'll learn why users around the world are making the switch to free, open instant messaging.
Spark is an Open Source, cross-platform IM client optimized for businesses and organizations. It features built-in support for group chat, telephony integration, and strong security. It also offers a great end-user experience with features like in-line spell checking, group chat room bookmarks, and tabbed conversations.
Combined with the Openfire server, Spark is the easiest and best alternative to using un-secure public IM networks.