Found this article a while back and I thought I'd share it with you all:
Best and Worst Abdominal Exercises
It describes some research done by the Biomechanics Lab of San Diego University to find the most effective ab exercises. They got a bunch of subjects to do 13 different ab exercises and measured the amount of muscle activation while doing them. This allowed them to rank the exercises.
One of the best exercises, they found, is the bicycle (on the ground making a bicycle movement with the legs, arms behind head and lifting the elbow to the opposite knee as the leg reaches the top of the cycle). I started doing this after I read this and I am pretty happy with the result.
It doesn't say here, I think, but what they found is that the best exercises have sideways motion and require stabilization. Some of the traditional exercises, like situps, do very badly partly because a lot of muscles other than the abs are activated. Situps on a stability ball don't activate the abs that much, but they only activate the abs, so they are one of the most efficient.
They do so that they found quite a lot of difference between the subjects, but it is all useful stuff anyway.
Actually the whole site is very interesting.