The latest version of Google Earth (4.2), which adds a feature to look at the sky, has a hidden feature as well. Marco's Blog reports that a flight simulator is now included in Google Earth.

To enter the flight simulator, press Ctrl+Alt+A. Here is a list of commands. There is supposed to be joystick support, and possibly force feed back, you can also use the mouse to control Aileron and elevators.

I tried it out for a few minutes, and I could see where it could get addictive. It's not MS Flight Simulator, but it is free, and doesn't take up gigabytes of storage space on your HD. Also, it's a little easier to hide on your work computer ;)

Download Google Earth Here (Just click agree to the ToS).