Keyhole is a beautiful piece of software, but it costs $40 a year to use. I enjoy staring at satellite photos of the earth projected on a rotatable, zoomable globe, but not when it costs me money.
NASA has come to the rescue with WorldWind, a free piece of software that does essentially the same thing. Their servers are swamped right now, so I’d offer up a review if I could get it to download faster. In any case, it’s exciting to see a free alternative out there to Keyhole!
If you’re more interested in zooming around the galaxy than you are in spying on our home planet, Celestia is for you.