Acrossair has developed an amazing augmented reality app for the iPhone 3GS, New York Nearest Subway which helps you find the nearest subway lines as you move and tilt your iPhone.
When you load the app, holding it flat, all 33 lines of the New York Subway are displayed in coloured arrows. By tilting the phone upwards, you will see the nearest stations: what direction they are in relation to your location, how many miles away they are and what lines they are on. If you continue to tilt the phone upwards, you will see stations further away, as stacked icons
It is based on the Nearest Tube app that they developed for London underground.
via Daring Fireball