Google's Geo Developers Blog recently highlighted a cool map in its Map of the Week feature. This map is a brilliant display of San Francisco's famous hills. Roads that go uphill from the location of the marker are red, streets that go downhill are blue. The more opaque the color, the steeper the hill.
Comparing the second screenshot to the first, you can see how the colors change as your starting elevation edges closer to sea level.
Even better, you can zoom in, turn on satellite view, and clicking will show the grade of any street.
The applications for this are interesting for tourism in San Francisco, but locally it would be cool to see this used in bike wayfinding.