Nasa World Wind is an amazing program that allows any user to zoom from high altitude to any place on the Earth's surface. The data comes from high-resolution LandSat and USGS data.
Some of images are literally jaw-dropping; in particular the USGS high-resolution ortho-imagery. (To view this, use the Layer Manager, select "High-Resolution Terrain Mapped Imagery" and deselect all but "USGS Urban Area Ortho-Imagery".) Several major US cities are covered at 0.25m. Good examples are Washington D.C., Houston, Chicago and Tampa Bay. For a list of ortho-imagery data see here.
The final cool part to World Wind is that it is implemented in C# and includes full source code.