This guy hooks up a modified mountain bike to his computer and connects it with Google Earth so when you peddle it simulates a flying experience. He takes a trip out over the Golden Gate bridge to demonstrate.
Wow, this is a great Idea, being a programmer myself this video makes me want to go get a mtn biken and create some custom controls for google earth myself and lose some weight. Doing it with multiplayer would be amazing and better yet, add jet fighters and allow us to blow stuff up!