The US National Guard has just released Drop Defender, a 3D Augmented Reality game where you fly your helicopter through 3D environments while taking the lead on rescue missions. You will also need to get the AR Marker from http://www.ardropdefender.com/ to play the game. The game itself (and the marker) are available for FREE from the Android Market. Do read the full description of the game in the market to understand how the game works with the marker.
Drop Defender (FREE) - Android Market