One who pretends to act like they are listening to what you are saying to them while they are actually just thinking of a response. These bastards have to follow every single clause that comes out of your mouth with something to the effect of: "Oh Yeah?" "Ahhhhhh" "Oh Alright" "I see" or one from the plethora of others.
I am really sick of the active listeners in this office. He knows exactly what you mean before you even say it... apparently.