Twist exercise. From "The New Rules of Lifting".

Muscle group: Abdominal

Steps

SETUP: Lie on your back on a Swiss ball so your head is level with or slightly lower than your hips. You want your feet flat on the floor and spread apart for balance. Keep your glutes and hamstrings tight. Cross your hands across your chest.
LIFTING: Use your abs to pull your upper torso up as high as it’ll go.
LOWERING: Slowly lower yourself to the starting position.

All exercises