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.
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This is cool.
Printing to PDF from Safari is pretty close to doing the trick. Safari seems to overlap the images slight when sending them to printing, so it's not perfect. But it still gives a nice map right there on your drive.
In Safari 3 I could only get a PDF with 8 or so pages - basically the first column of what would be a grid of A4 pages.