Hartl's main project in his Rails book.
NOTE! READ THIS! You can't sign up on that site. I didn't configure Heroku to send emails and the app needs to send an email when a user signs up. So don't sign up. Seriously, Heroku will complain and will throw an error. BUT, if you are looking at the code as an example, it all works the way it should.
However, if you'd like to log in to check out the site, you can do that with the email example@railstutorial.org and the password foobar.