I consider that the most useful and often the most interesting wallpapers for your computer desktop are images that are relatively feature-less. A picture full of little details continuously grabbing your attention is a major nuisance. It’s much better to have either a very smooth image or a photo containing a lot of continuous tones.
Look at the full set of the Windows 7 wallpapers. This is the upcoming version of Windows (after Windows Vista, it seems that Microsoft intends to switch back to a numbering scheme). Most of them may be colourful, but with very smooth surfaces where your icons will be appearing quite neatly and they offer a nice contrast.
So, I was quite interested when I stumbled upon the collection of pictures taken by the HiRise (High Resolution Imaging Science Experiment camera installed on board the Mars Reconnaissance Orbiter). Shooting photos of Mars surface, this photo camera brings extraordinary color images that provide nice patterns to be used as background for your Windows desktop (or even Linux or Mac desktop) and can be renewed quite regularly: Their catalog is available on the University of Arizona web site.
Furthermore, they provide an enormous resolution for their images which is a very good way to extract any size you may need for your extra-high-resolution background or to cover your 2- or 3-LCD display. Or even more. If you feel that there own selection of wallpapers is not enough:
You can still stick to the original size (JPEG-2000 format images range between 0.5GB and 3GB).
And the good news is that there is no copyright restrictions, so you could do pretty much what you want with them: Really free desktop wallpapers.