I've always either done the standard planking or crunches but I came across this on shape.com so I'm thinking I'm gonna give it a try the next time I hit the gym :)
The Best Abs Exercises: Side Plank
Why it works: This abs exercise is more challenging than a traditional plank because you're supporting your entire body weight on two points of contact instead of four. As a result, you must work your core harder to stay stabilized.
How to do it:
A. Lie on your left side with your elbow directly beneath your shoulder and legs stacked. Place your right hand on your left shoulder or on your right hip.
B. Brace your abs and lift your hips off the floor until you're balancing on your forearm and feet so that your body forms a diagonal line. Hold for 30 to 45 seconds. If you can’t hold that long, stay up as long as you can and then repeat until you’ve held for 30 seconds total. Switch sides and repeat.
There's 2 other moves but this appealed to me most because I'm familiar with doing the plank. Full article here.