This is a little Sinatra + MongoDB/Mongoid + Stripe app I've built for my own use to allow people to register and pay for kung fu courses I organize. I have not written it with anyone else in mind, so you will definitely need to modify it if you want to use it for your own purposes, but at the very least, it might serve as an example or educational app.
Also, please note that the design is built on SmartStart, a non-free design purchasable on ThemeForest. I will happily allow you to use the customizations I built on top of it, but if you want to use it, you should be a good internet citizen and buy your own version. (Come on, it's only $15!)