Do you have more than one IM client? AIM, Yahoo!, MSN, Live, Facebook, MS, or Gtalk? Then you should try Meebo! Clicky the pic above and it'll direct you to Meebo.

Meebo allows you to be on all these client all at the same time. You don't have to download any software and it's free to join. Once you sign into your meebo account, it signs you on to all the IM accounts that you added to it. You can even sign on invisible if you like.