Hidden inside of Google Earth's version 4.2 is a flight simulator, which allows you to feel like you're piloting an airplane over satellite images of the Earth's surface.
To access the hidden feature, open Google Earth and (if you're using a Mac) hit Command+Option+A (note it must be capital A) and (if you're using Windows) Ctrl+Alt+A.
You are offered a choice of flying a Viper fighter jet or a four-seater plane.
Here's a one-minute video of what this looks like over London:
Thanks to Marco for posting the tip on his blog.