Recently some of my friends in the physics department drew my attention to one of the coolest programs I've ever seen. The program, World Wind, by NASA, allows you to interactively view the Earth. The program connects to the NASA server, which contains a massive, multi-terabyte database of satellite imagery, which feeds into the program. The result is that you can hold the world in your hand, spin it around, and zoom in all the way down to an incredible 15 metre resolution. I should emphasize that the entire Earth is mapped in the database. In addition to the satellite imagery, the database includes topographical data and can use it to reconstruct elevation profiles. There are also facilities to superimpose city and suburb names, national borders, and even animations of weather patterns. Be warned, however, the program is big (around 250Mb) and you'll be needing a pretty speedy internet connection to get data from the NASA servers (which have been down a lot of the time due to overload). Unfortunately, the program is only available for Windows.
Below is a snapshot from World Wind, taken looking over Mt. Cook on the south island of New Zealand, where I'll be climbing in a few weeks time. For this picture I enabled elevation profiling to reconstruct a 3D view. A pretty remarkable program, and completely free!