This is a simple Sinatra app that shows how to use Restforce with Force.com canvas apps.
git clone git://gist.github.com/4052312.git sinatra-canvas
Now create a new Connected App on your Salesforce instance. Point the Canvas App URL to https://localhost/canvas and chose "Signed Request (POST)" as the access method.
In one terminal, startup the app:
CLIENT_SECRET="secret from above" bundle exec rackup
In another terminal, startup an https proxy:
sudo bundle exec tunnels 443 9292
Open the app up in the Canvas App Previewer, and you should see your app running.