BBC Future has provided us with a colorful application for twiddling with the variables in the Drake Equation. The Equation is a thought experiment meant to give a very rough idea as to the number of inhabited planets and technological civilizations existing within the Milky Way galaxy. This version is divided into four sections, allowing you to see how many potentially habitable planets exist in the galaxy, how many are inhabited, etc.
(This is only an image. Go to BBC Future for the application.)