A big map of the world, using Google Maps' new Terrain images:
Warning: high zoom levels take a while to load and use a lot of memory.
Now all that's needed is a way to save the whole page as an image: neither Save As Image, Pearl Crescent Page Saver or Print > Save as PDF seem to be able to handle the large image. Update: Screengrab! works perfectly.
2008-01-10: Updated to include Moon, Mars and Sky imagery.