Paper prototyping is a technique used by software designers and user experience engineers (like myself!) that can be superior to "traditional" prototyping and testing for a couple of reasons:
1. You don't need to spend the man hours writing code when you can draw it
2. Because the designer is running the app, they become more aware of user actions and mistakes. Users also, must make their actions and thought-processes more clear when navigating the app, so that the designer can follow them. This can provide valuable insights that interacting with a computer prototype cannot.
Check out this awesome video of paper prototyping in action:
Sometimes the "low-tech" solutions really are the best!