Virtual Earth allows you to zoom in on a map of the U.S. until the map turns to aerial photos. You can zoom in to an impressive level of detail, enough to distinguish individual trees. You can even, if your curiosity inclines this way, see photos of Nevada's Area 51.
If you happen to have some old 3-D glasses lying around, take a look at the USGS' 3D-formatted photos of Lodore Canyon, including many shots from Powell's 1871 expedition. The parent site has 3-D images of many other national parks.
And if you're really obsessive about this kind of stuff, you might enjoy these writings on prominence theory and the mathematics of orometry. Prominence, if you're wondering, is a method of measuring a mountain's size and regional importance independent of its absolute elevation. Imagine it thus: picture sea level beginning to rise around your favorite mountain. At some point, the water would cut off your mountain's land connection to any higher peaks. It would then be the highest peak on its own island, and its height above that imaginary sea level is its prominence. Enjoy!
