Apart from the electric lawnmower, the NumLock key, and the perm, NASA hasn't produced much that has been useful to mankind. But that has changed with their Open Source World Wind software, which anyone with even a vague interest in geography would love. It lets you zoom in from outer space down to ground level, to view cities, streets, and even cars in close up. Even better, height data is used, so mountains and valleys are realistically portrayed in full 3D.
There are lots of different data sets to choose from. The most detailed is the USGS urban area ortho, but it only has data for some US cities. Community LandSat (Pseudo Color) seems pretty good for many areas. NASA is busy processing more data, so higher resolutions should be available soon.
In use, the software downloads large amounts of data, so it may be a little slow if you don't have broadband. On my 6Mb/s connection I have to sometimes wait up to a minute for more detailed landscapes to be drawn. However, it can be strangely mesmerising watching a mountainous landscape such as New Zealand being gradually drawn in increasing realism.
I have demonstrated it to my family and other relatives passing by, and they have been universally impressed. And the fact that this is a free gift from NASA makes it a guilt-free experience.