Sinatra is really fun to work with. It's small and fast. It doesn't make many assumptions. If something goes wrong, it's pretty easy to go into the source and figure out what is going on.
There were a couple projects I wanted to take for a spin: Sinatra, Datamapper, HAML, and SASS. I decided to roll them all up into one proof-of-concept project. I don't go into a lot of depth on each, just enough to know that I can get it all up and running.
Also, since I'm running Passenger on some production boxes now, I wanted to deploy my Sinatra/Datamapper app through Passenger's Rack support.