I do lots of things for abs.
When I was in training for the Marines, they had us simple to (up to) 100 crunches in 2 minutes.
You sit with something/someone holding your feet down, bring your heels to your ass, keep your arms crossed in front of you, and crunch until your arms hit your thighs. Drop, repeat. Do as many as you can in 2 minutes, with your goal being 100 (or more)
I would also do:
decline situps with a 40-60lb weight on my chest at the gym. Careful with these, as they can be stressful on the back.
bicycles, or weight assisted bicycles at the gym.
leg lifts at the gym. (knee ups, legs out, knees left, knees right)
dumbbell oblique lifts, sometimes hanging outward from one of the machines.
On top of that.. running, swimming, and a good diet.
On a side note:
I came across this pic when I was googling some of the exercised to see if I got the name right.
I'm not normally one for muscle-chicks, but this one is kind of hot
