Best ab workout with no equipment required

Read: No exercise ball required. My tiny apartment does NOT have room for a bright purple bouncy toy. Besides, it might damage some of my priceless antiques, like my stove.
I like ab workouts that don't require anything but your six pack and maybe a yoga mat or a rug. And here's a great one I found in out local paper.



After just five repeats of the continuous crunch, I was feeling it!