Google Earth comes to the iPhone, and it is good
I was hooked on this app from the first time I made Google Earth zoom in on my location. The visuals are stunning, and you can activate Street View once you're zoomed in. It literally took two clicks to get a view of my front door: one click on the location button, and one to allow Google Earth to use the GPS data. There's also a compass button to point you North, and a search function that even includes your Address Book contacts. The only thing missing is a tilt mode: it would be fun to explore the earth by using the accelerometer, but it wouldn't be very practical.
UPDATE: Google Earth does have a tilt mode! You can turn it on and off in the preferences. Thanks, Belden.
CORRECTION: Street View isn't coming until the iPhone 2.2 firmware is released. The satellite image of my apartment is just at an odd angle and looks really good on an iPhone screen. Thanks, Steve.