Proteus is an application with which we can communicate simultaneously with our MSN Messenger, Live Messenger, ICQ, Yahoo!, AIM and Jabber contacts, from a very simple interface (that integrates perfectly with Mac OS X). The program is totally written in Cocoa and, furthermore, it hardly uses any system resources.
The program's configuration is very intuitive, due to the fact that we'll only have to add the accounts of the several services. Once all this is prepared we'll only have to start chatting with our friends and family.
Despite how easy it is to use the application, it lacks some features that come in very handy and that are included in some similar developments, like the management of conversations by means of tabs, the compatibility with plug-ins that improve its features and the possibility to share the desktop.
If what you're looking for is an instant messaging client for various services, that is as light as possible, Proteus may be a good option.