I'm sure Google bought Keyhole for a song, but i'm glad they did because I otherwise wouldn't have tried this app. (With GOOG at $300, I'm sure Keyhole got a lot more out of the deal than they expected - good for them)
Google Earth allows you to get satellite views of wherever you are going on the globe in 3D.
What is this useful for? Well, I wanted to get a better feel how far my hotel was from the airport on an upcoming trip to Spain, and well, this was the tool to do it with.
I'm sure there are other uses as well that I certainly don't need or haven't thought of, but color me impressed with just this.