Other than the Ab wheel that Pandaemonium recommends,there is this. While lying down (bench or floor...knees bent...feet held firmly down), hold a BB above the head (like completing a BP) and begin crunches that way. A few guy's will come up all the way to a sit up position, but not needed. Different leverage shift on the ab area. Can be a tough exercise when more weight is added to the bar. Or hold a BB/DB, arms stretched out like doing a straight arm pullover, and try crunches. Different leverage again.
My view only...that's all: Ab's should be treated with-in a regular BB'ing rep scheme (8-12). 20 reps or more, of endless sets, is just spinning your wheels.Twice or three times a week should be more than enough. Ab's are a very strong muscle structure and can be trained that way. If you want thick obliques than do side bends by all means. Sports training uses a form of this oblique work (over worked term..core strength). One of the better ab (or even legs) movements is working on a true Roman Chair. Which is kind of rare these days. What people sell as RC's are not always that. Good Luck.