Here are two instant messenging clients that I have found to be useful.
This is a purely web-based IM client, which allows for a windowing environment within a browser. It supports AIM, Yahoo, Google Talk, and MSN. This is great for situations in which there is no local IM client available (or it is acting up).
This is an open source MSN client that has been compiled across platforms. It appears to be well organized and have full functionality. Amazing, it supports video webcams on the Mac OS version (whereas the MSN Messenger for Mac OS from Microsoft does not!)