Another feature I use all the time on the iPhone is Google Maps...I was going to use the term "App"...but it is more of a navigation system that is a just as much a basic function of the iPhone as the phone itself. I use it so much that one could argue that I carry a navigation system that has a phone. I mostly use it to find my way around on foot and on mass transit.
The mass transit interface, in my opinion, has revolutionized the experience of riding the bus. I recently put together a slideshow on Planet Green detailing this function:
Commuting on Mass Transit with iPhone 3GS
Photo by Trevor Reichman