Pow: Zero-configuration Rack server for Mac OS X: "Pow is a zero-config Rack server for Mac OS X. Have it serving your apps locally in under a minute."
Holy awesomesauce. Build (or clone from github) a bunch of Rails or Rack apps on your local machine, run them with a tiny server using real hostnames based off of symlinks.
This is just one of those tiny "why didn't I think of that" things that makes 37signals so good.
(Via Daring Fireball.)