First Google Earth, now Google Mars. Some beautiful pictures and nice links to interesting spots on Mars.
Another one of our favorites from NASA Education are the Toys in Space videos. The free real player videos are a little grainy, but we received free copies in the mail a while back, and they have been great to watch with the kids. In the Toys in Space episodes, shuttle astronauts took common kids' toys (clackers, wind-up toys, gravitron, etc), and looked at how these toys behaved differently in space. They make nice lesson plans for observations, analysis, and prediction - particularly if you have the toys at home or at school... why do things do what they do, what unseen forces hadn't we considered in what we see, and what might we predict based on what we know?
Toys in Space II