I have multiple personalities. This little app lets me switch between them, or tell everyone on all my lists that I'm away. So much easier than iChat which makes me launch a separate window for each list, and only supports signing in from one account at a time (like one AIM account at a time). The duck is a little annoying at first, but it kinda grows on you.

All in One Messenger for MAC (aim, msn, yahoo, jabber) with lots of fun extensions to customize it.