Vups (tough)
Planks (full, hop outs, sides, elbow)
Mountain climbers
Sit ups
Leg lifts (figure 8s)

If your working to your abs to pop focus in hitting your entire core (abs, hips, lower back and butt). Your entire core is interconnected, so focus on working out the entire area. If your hips or lower back are weak your stomach will look bigger than it actually is.

I built my full core workout (Core Blast) from exercises I found here and at ab-workouts-guide.com (it's full of ab workouts that you might find interesting).

Good luck and God Bless!