Dancey trancey songs that start off slowish and gradually build up are immense. Pendulum, Basshunter...I discovered Paffendorf a while back, a couple of their songs are AMAZING to work out to, especially Where are you and Lalala girl.
Generally most fast songs that you listen to at a high volume can be really motivating...I definitely could not live without my ipod at the gym. One time my ipod ran out of battery while I was on the cross trainer...I literally almost died of boredom and couldnt actually make myself stay on it much longer!