I hate writing boilerplate code, and since I couldn't find any good templates for getting a basic Sinatra app running, I decided to make my own.
The Sinatra/DataMapper template includes basic code to set up a database, a simple user login system, a little bit of HTML/CSS, and other things that I usually include in my Sinatra apps.
Everything here is in the public domain, so do whatever you want with it! Also, feel free to fork this on GitHub to help improve it.