It's not mac-specific, but I like the Hubble Wallpaper page. If you have a widescreen monitor you'll be happier with the results if you crop them first in iPhoto. Then just use System Preferences--Desktop and Screen Saver to select the picture you want. You can even set them to change every 30 minutes or so.
There's also http://www.macsnaps.com -- some of the postings there are desktop pictures, others are screenshots or photos of computer setups. Lately several, er, exciting images have been posted there.
Absolutely AWESOME pics. I'm using the feature in Panther that will rotate whatever pictures you put in a file and use as background. Favorites are the Aston Martin pictures, just gorgeous. Cheers