I'm sure there are dozens, but this one is free...you can have 5,7.5,10 minutes basically just going from one exercise to another, no frills. It is by Denise Austin and of course it is a pitch to do her weight loss get healthy deal, but I just move past that. You can get in a great micro workout for your abs in 5-10 minutes.
Anyway, just passing it on...I've been using it for a few days...really nice quick daily workout. It downloaded as Ab Workout on my iPhone.
It is just a woman lying down doing the exercise for 30 seconds to a minute, depending in how long you choose.