This free game uses Google Earth to play a kind of geocaching, where you launch the game from within Google Earth and follow the clues to various location checkpoints. If you find the envelope “hidden somewhere on Earth, you will advance to the Big Game.”
I haven’t played it myself yet, but I may give it a try in between actually trying to get my GIS work done.
For a basic tutorial, check out the Earth Contest website
Via Ogle Earth