Another great ab exercise. You need to be in control of the whole movement to get the maximum effect.
Lie on a bench and hold on to either side of it by your head with your hands. Lift your legs in the air and then crunch up, lifting your bum off of the bench. Stay slow and in control the whole time. You can start with your knees bent to make it easier, or put the bench on an incline to make it harder.