The skull ride:
Essentially a reversed knee-on-belly pin, this is an excruciatingly painful position to be pinned in. This is not even a submission, mind you, just a place to stop while you decide what limb to attack...however not a few will want no more once they feel your full weight bearing down on their helpless head. It doesn't work well without pants to grab onto, so you won't see it in any cagefights. Nonetheless if you are looking to inflict pain on a downed opponent I heartily endorse* the skull ride.
*I even more enthusiastically recommend that you avoid having someone practice it on you. That sucks.