You've got to love Google - and got to love mashups. In this case it's a clever use of Google Earth's interface, accessed via a web browser page, and with very basic (left/right) controls for a simulated fighter jet.
And you can't crash. Or go up or down. But hey, you can type in any location on earth (such as your home town) and fly around the area at (simulated) great speed.
Very cool. Don't worry about the meaty browser plug-in, by the way, I'm sure other mashups will be along shortly to make further use of it 8-)