One of my favorite places on the Internet is Digital Blasphemy.com. This site is by far the place to get desktop wallpaper. I’ve been a fan for about 4 years and with each update, I am more and more impressed with Ryan Bliss’ work. He is an amazing digital artist.
One of the coolest things to do is go look at the 2001 work and compare it to the current offerings. With every new version of software (Truespace 3D, Lightwave, etc), Ryan manages to push the envelope and get the most out of new features. This goes to show you that a true artist can get the most out of his tools.
Digital Blasphemy has free section, but I would recommend that anyone pay $20 and sign up for a 1 year subscription. The paid wallpapers are well worth it, especially if you spend as much time on the computer as I do.
I went to DigitalBlasphemy.com and looked at the ones he had for free, and a lot of them look like the ones on the site I use for backgrounds.
DeviantArt. They are all free and they have various pieces besides digital.