Yahoo! has a new service called Map Mixer. You can upload an image of a map of just about anything and overlay this onto a Yahoo! map. There is then an opacity control to show more or less of the uploaded map. This ultimately allows people to tie their local areas and local mapping needs into a broader context.
For example, see the map of University of Southern California. This has far more detail of the campus than a normal Yahoo! map would ever show, but now you can also see how to get to the campus and where it’s located geographically.
You can also show a satellite image for the background map. This is particularly interesting, and it could be great for hiking maps and such. Very simple idea, but also very cool.