TrailTopo is a site dedicated to providing free maps. It uses services such as Google Maps, MyTopo, and eventually more. You can generate U.S. maps with a click of a button, just scroll to the place you want a map for and generate nicely detailed maps. Keep in mind that TrailTopo currently does not generate topo maps outside of the United States but does display terrain mode for the rest of the world.
Although I (Josh Lewis) put this site together and figured out what I wanted for the map generator, non of this would be possible without a few organizations and the help of individuals who helped me out. Whether it be personally or by developing community software.
MyTopo - The main source for my maps when using the map generator button. They also are the same source for the topo displayer. I have verified with MyTopo via email that what Alpine Topo does is fine according to their T.OS.
Google Maps - The entire map displayer is completely based on Google Maps API3 which is pretty much the base of the map displayer. So without Google, the Map Generator is completely impossible.