NASA World Wind is a truly excellent Open Source, free 3-D world viewer, incorporating multiple layers of satellite imagery and topo data, as well as place names, boundaries, and geographic features. The mouse-driven interface lets you zoom, tilt, and pan the globe, giving the user a ‘fly-through’ effect from the 3-D-mapped ortho-rectified imagery. It’s a pretty big download at 256MB, and it does auto-download more content if you request it to, but it’s well worth it.