($0.99) - I probably shouldn't be posting this, but it's such a freakin' amazing idea. I can't pass it up. It even has a speed-activated mode that uses GPS to calculate when you're nearing 88mph (at which point, you will see some shit). Don't ask, just buy.
EDITOR'S NOTE: Don't actually drive that fast, kids. You can adjust the activation speed in the app. If you do drive that fast and don't hurt yourself, I'll hurt you.