Elise Gulan has very effective pilates inspired workout for abs made in cooperation with Exercise TV. You can pay for it and download in minutes.
As I have gotten too fat to keep up with this workout so I stick with Tony Horton's - Great Body Guaranteed Great Abs Workout, only 7 minutes and you are done, very effective.
I also like Tamilee Webb's I want that body. There are 2 different level workouts for abs, arms and buns. Each workout approximately 15 minutes.