Imguana is a multi-protocol instant messenging client (similar to Gaim) for Windows. In the future, a Mac and/or Linux version may be released.
PlanetaMessenger.org is a Instant Messenger with support to all well known instant messenger networks including ICQ,MSN,AIM,Yahoo! and Jabber through a powerfull plugin engine. PlanetaMessenger.org support multiples look and feels, plugins and profiles.
The PlanetaMessenger.org Libraries project is a set of open source instant messenger libraries used and maintained by PlanetaMessenger.org. You can find protocol implementation for MSN, ICQ, ComVC networks for use on standalone and server applications.