Honestly, it's hard to pick just one.
Banking apps (I know some banks exist, just not any of the ones I use), MLB At Bat, Nook, more video streaming apps as mentioned, and many others.
If I had to choose just a single one though, it would be Zipcar. Having an app for that service would make my life much easier. (I know that I can use the mobile site, but that can't use a GPS to find cars near me, and it can't be used to unlock the doors, etc.)