Like Google Earth, but with fewer roads so far (though, you can check up on the locations of Spirit and Opportunity): Google Mars! Check out both an elevation-shaded and an actual image at an impressive range of zooms. At this point only covers a limited chunk of the western hemisphere. They hope to tie it into the Google Earth client in the future to allow fly-throughs and such.
Glimpses of the websurfing habits of a computer science professor, obsessive crafter, occasional gamer, and all around geek