The newest version of Google's 3D Earth mapping tool not only includes Google Sky, sure to be a hit with astronomy dorks, but it also contains a hidden flight simulator. Only two aircraft are currently available in Google Earth 4.2's flight sim (an F-16 jet fighter and the Cirrus SR22 prop plane) but there are plenty of airports and real world locations for one to fly around.

Not a bad deal for zero dollars.

Google Earth Flight Simulator [Marco's Blog via Google Blogoscoped]