MirandaMiranda IM - Open Source
Miranda IM is a small, fast, easy to use instant messenger which supports multiple protocols and many UI and service features that can enable users of all knowledge levels to easily unite all their friends, accounts and chat sessions inside one app. It is very fast and uses very little system resources and requires no installation and can even be made to fit on a single floppy disk or USB drive and run from them without installation on PC. By default Miranda IM comes with only a few basic features, but its functions always expand by the adding of (more than 350 by now) plug-ins that cover everything from simple visual tweaks to large functionality changes like adding of new chat protocols. This is what makes it the right choice for the novice and the advanced users alike.