Life is too short for long URLs! So it's a URL shortener, implemented in Ruby on Rails.
It's pretty basic. It uses Rails' SecureRandom interface (moved from ActiveSupport to core in 3.2) to generate a 6-ish character URL-safe Base64 encoded string alias for your long URL and checks that it is unique.
Short.ly was developed as part of General Assembly's awesome Rails for Developers course, the content of which is available on GitHub.
Short.ly uses MongoDB, strictly for the fun of it. So you'll need to install that, the easiest way for Mac probably being via Homebrew:
brew install mongodb
And mainly because of its name, we're running the Unicorn* app server. Just type unicorn in the app root, or mimic the way it gets run on Heroku by typing foreman start - you will need the Heroku Toolbelt.
(* okay, it's fast on Heroku too!)
The rest is pretty self-explanatory, and life's too short - so just enjoy!