a dbell chest press on the stability ball. i do mine for time and as many as i can do (palms facing in). i think i got up to 25 or 27 or sso in a minute with the 50's. i use it more as an endurance/speed tool. the benefit of doing them on the ball is you can get a little help out of the bottom so its possible to take yourself further into fatigue. pretty damn taxing if you ask me.
Effective for inclines and flys as well, and good for core strength if you use them for dbl shoulder presses. I use them to supplement my regular program, never to replace my heavy presses though.