If you use AIM (America Online's Instant Messenger), you should give Adium a try. This Cocoa replacement for AIM has a nice feature set, is completely free, has source code available, takes up minimal screen real estate, and is updated regularly.
Thanks to WCityMike for reminding me about this great little program!
Mac OS X Hints
http://hints.macworld.com/article.php?story=20011204213025642