deviantART is much more than a free wallpaper site which you'll soon find out can be both good and bad. You'll find very unique and original free wallpaper at deviantART but finding them and adding them to your computer desktop may prove to be quite a task.
HDwallpapers.net is a free wallpaper site with a lot of HD wallpapers that look great and are over a wide variety of different subjects. The ideal resolution for your device is automatically selected or you can use the drop-down button to choose a different one.
Your favorite wallpaper site from not too long ago, no discussion of wallpaper sites is complete without a stop by Digital Blasphemy, run since 1999 by one man (and friend of Lifehacker!) Ryan Bliss. Its beginnings go back to the early web as one of the first places to find really unique and interesting 3D art and unique wallpapers that weren’t photos.