I would like to present to you links to two great wallpaper sites that have almost no advertisement on their sites, unlike many other wallpaper sites that seem to bombard you with ads and popups. The first is joejoe, a nice site if you are looking for nature wallpapers. He has some pretty interesting close-ups of animals and plants that you can download to your local system.
The second bigger site is topwalls which has more categories and more wallpapers in total numbers. There seems to be a focus on entertainment wallpapers especially games and movie ones. If your desktop needs a change pay those site a visit.
Update: Both wallpaper sites that were listed here are no longer available on the Internet. If you are looking for wallpaper images, check out the following alternatives that offer thousands of images each. All three let you download wallpapers directly without registration.
Desktop Nexus is one of the biggest wallpaper repositories on the Internet, with more than 800,000 wallpapers listed on the site. The background images are listed in categories such a animals, anime or sports, and the wallpaper downloads are even automatically optimized for the screen resolution of the computer that you are using to connect to the site. All wallpapers can be downloaded free of charge, and without registering on the site or becoming a member first.
HD Wallpapers is another high quality site that is offering free high quality wallpaper images. The site displays wallpapers in categories, and it is possible to download them right on the site without registration.
The third wallpaper site alternative is Interface Lift which is making available thousands of wallpaper images. The selection of different resolutions is one of the largest on the Internet, with mobile devices and even dual monitor wallpapers offered on the site.