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