Adam Ford's Swiss Ball abs is fabulous! 3, 20-minute ab workouts (with warm-up and cool down), in three phases. Phase one is intermediate/advanced (but the intermediate moves are often modifiable to advanced); phase 2 is advanced; and phase 3 most advanced (you even do a superman ON THE BALL in that one..though only hold for a second or two). BOth abdominal and back moves are done, so the entire core is worked.
The instructor is a former professional cyclist, and his current physical therapy (neuromuscular therapy) work really shows in the form cues he gives. And since all he wears is a pair of blue boxers over some short black bike shorts, the eye candy factor is rather high...and you can see the muscles at work.)
I got mine at amazon.com.