World Wind allows any user to zoom from satellite altitude into any place on Earth, leveraging high resolution LandSat imagery and SRTM elevation data to experience Earth in visually rich 3D, just as if they were really there.
World Wind has a full copy of the Blue Marble, a spetacular true-color image of the entire Earth as seen on NASA's Earth Observatory: the Blue Marble.
Put together from data of a variety of satellites such as MODIS and Terra, the Blue Marble can be seen in all it's glory at 1 km per pixel resolution.
Whilst it's not cutting-edge 3D, it's still very impressive and also good fun and educational, too. Being open-source (using .NET) you can also get the source-code too. It also has lots of extra features, too:
http://learn.arc.nasa.gov/worldwind/features.html