As somebody who travels a lot, Yelp is a great boon to me when looking for a place to kill some time or grab some grub. It accesses a user-maintained database containing reviews, GPS location, and more, as well as provides directions from your current location to the destination of your choice. It is an extremely useful and easy-to-use application.
This is not an easy app to get a hold of - I have never personally used it - but it looks awesome! The vehicle is based on MIT's famous quadrocopter, capable of insane stunts and maneuvers, but being able to purchase one and control it with the gyroscope on an iPhone, whoa! Count me in (as soon as I'm rich!).
This is probably the most-used app on my phone. It simply turns the camera flash on, enabling it to act as a flashlight. On my phone, the camera flash is quite bright, so it is an intense flashlight always available in my pocket. It's not very advanced or spiffy, but since it is my most used app, I figured it was worth a mention.