This is the second application for the Ruby on Rails Tutorial by Michael Hartl.
This app simulates a simple microblog. You can create new users (the app requires you to enter a name and e-mail) and new microposts (which need to have a valid user id attached to them, and cannot be longer than 140 characters long - just as Twitter). In order to create microposts, your URL should be .../microposts instead of .../users.
You can see it live here.