Here is a pretty cool ab exercise that I do at Karate and it really gets the abs burning without crunches. Thought I would share.
Sit on on floor with your legs out in front of you. Lean back slightly using your arms to support yourself. Raise feet about 6" off the floor. Next, keeping feet together, try to touch your heels to to right butt cheek, then back out in front. Next your left butt cheek, then back out in front. It doesn't take long to feel the burn!"