Real Time Satellite Tracking is a Google Maps mashup that takes the current position of many orbiting satellites and plots their locations directly on a Google Map. You can check on the location of the satellite that your DIRECTV service is connected to, and see where The International Space Station and Hubble Space Telescope currently are. It has listings for many different types of satellites such as GPS, Military and Amateur Radio along with all kinds of information about each orbiting object. I'm amazed at how fast they move!
One of my favorite Google Maps mashups is the Dartmaps mashup which is very similar to this. It displays the real time locations of commuter trains as they chug down the tracks in Dublin, Ireland. These, to me, are the best kinds of Google Maps mashups.. Real things, in real time.. On a map!