1) Car Chat: You remember CB radios? Let's replace those with smartphones as well. Download the app, pick out your handle and put it on a bumper sticker. Tag your account with the make, model, and color of your car. Now you'll be able to say "DackBird29, your tire is flat." or "Blue Dodge Caravan, your tail light is out" and the other driver will actually be able to hear you. And if someone starts ranting like an 12 year old on Xbox Live, then that driver can be banned from using the service. It can even have a Reddit-style karma system for when you provide roadside assistance to fellow drivers. Altruism is great and all, but it's just so much better when you get recognition for it.
2) The Bike Clip App: I got a bike clip for my phone, and it has a nasty habit of pushing the volume control and the camera button on the side of the phone. So I have a lot of accidental pictures of the front tire of my bike that I have to delete. So it would be fabulous if I had an app that turned off the side buttons when I have my phone in my bike clip.