Best ab exercise bar none is pull-ups holding a dumbbell between your ankles. You won't see it in "get ripped quick" programmes because if you can even do it, you probably already know what you're doing.
to show how the abs can be worked, go to a pull up bar with a spotter. have them hold your legs bent while you hang directly below the bar. do 5 or 10 pull ups. now have them pull your legs back about a foot or more and do 5 pull ups. usually i can feel my abs strain pretty hard. kiping pull ups work a lot of your core but can be dangerous on temporary pull up bars attached to door frames.