Leg levers are a great ab exercise that you can do anywhere. Lie with your back on the ground and your legs extended. Keeping your legs straight, lift them 6 inches off of the ground. Slowly raise them to around 18 inches up and then back down to 6 inches. Repeat this motion in slow, deliberate movements.
Leg Levers Video
[youtube=http://www.youtube.com/watch?v=i14hO_dODkk&rel=0]