There's a great new variation of the plank exercise in this month's Men's Health Magazine (July/Aug 2009). Any version of the plank is a great ab exercise. It fully works you core.
Here's how you do it: Assume a push up position with your hands a little wider than shoulder width, body in a straight line. With your abs braced, walk your hands out in front of your head until your hips start to sag. Then walk your feet forward until your back is in the starting position of a push up. Repeat a total of 6-10 times.
Canadian researchers found that this move achieves 100% activation of your rectus abdominis (the six pack muscle).
More evidence of the varitey of great ab exercises that are not just crunches. Any of the versions of the plank are far superior exercises to crunches. They should be a main part of your ab training.