It has every home, not just homes that are for sale. It's GPS-enabled, so it follows you around the neighborhood as you drive or walk. And the map has pan and zoom functionality which is slick.
It's free. Check it out.
Here's the official Zillow video on it.
And with all due respect to my friends at Zillow, here's the much more entertaining video of my 4 year old playing with the app: