I have used an app called ID Release:
ID Release FREE - create, store and send model releases
Nancy Wolff is the legal counsel for this one, so I figured the language must be good. I have only had a chance to use it on a shoot once, but it worked well. The model had a little bit of a problem typing on my iPhone since he hadn't used one, but it was fine in the end. It emailed a pdf to both of us, which was convenient.
It was somewhat awkward waiting for him to struggle through typing in his information, but it's great having the model release on the spot.
Mars