What happens when someone has too much free time, programming talent, and a burning question? This.
Summary: HeyWhatsThat is basically a rather advanced map hack site which includes viewsheds linked with government/public name systems (names with xy data) so from a given point you can see all the major peaks nearby. The proprietor and designer, Michael Kosowsky, has steadily been adding stuff to it and it now includes pathing (which gives you an elevation profile), and a night view.
I'm moderately curious why Google hasn't bought him out/hired him/stolen the idea. As far as I can tell, Kosowsky built all the tools (minus the Google Map of course) from the ground up. It includes some interesting little quirks based on his response to user comments, like the way it handles the curvature of the earth and the fact it switches to metric if outside the United States.
(To be completely fair, you might have heard about this site. Several other bloggers, including Ogle Earth, have wrote about it.)