Update: Looks like ZeepMobile has shut down so the example app is no longer up. I've kept the repository though.
I was playing around with Sinatra a few months ago and decided to create a small app with it. It's a very basic online fuel efficiency tracker which I've dubbed with the stupid name: Fuelyo.
You send a text message to the app (I'm using ZeepMobile for SMS support) with the details of your fill-up. The app calculates your MPG and sends you a text back. You can then go to the app online and see a history of your MPG ratings.
I've put the source code on GitHub. Since this is my first real Sinatra app if anyone has any pointers regarding the code please let me know!