NASA World Wind

NASA World Wind brings a huge amount of NASA’s satellite imagery to your computer, allowing you to zoom from satellite altitude into any place on Earth. It also uses a 3D model of the earth so you aren’t just looking at flat images but have a real-life view of the topography you’re flying over.

Google Earth is a similar application. And of course, you don’t even need to download and install either of these free applications if all you need is a quick look, both Google Maps and Microsoft’s Terraserver offer satellite imagery of Earth.