Flabby Physics (on-line Flash game) - Molleindustria
Deform a shape with SPACE BAR, to bounce your ball into the star. Incredibly hard at times, but quite fun just to mess around with. High contrast graphics. Catchy kazoo music. That just about sums it up!
And just for your info, this came from the GAMMA IV competition that saw many more one-button games come to life. Not many were this good though.