You probably have your IM application set to automatically login to your different accounts as soon as it starts up, but what about when you want to chat without announcing your presence to the world.
Macworld details a simple keyboard shortcut that will launch iChat or Adium without logging into any accounts. The trick: Just hold down the shift key when you launch your respective IM app. Doing so will start the app offline. From there you can log into whichever individual account you want and control your status ahead of time. I tried this trick on Pidgin and Digsby to no avail and searched for similar shortcuts without luck. (If you know the proper shortcut, let's hear it!) It's such a smart and simple shortcut, and having been exposed to it, you can't help but wonder why all IM apps don't have this option.