In the StarCraft community these are known as "barcode users". They often tend to be professional players on another account, to ensure that they aren't recognised and their opponents can't learn their playstyle.

My friends AIM account back in the day was essentially this. He basically had to find you.

I used to use aim chat booters by a blog named 'esoteric code' program was called subterfuge. Familiar with it by any chance? I thought it was brilliant.