These are the five wallpaper sites that I use the most; they usually have a broad selection of photographs or digitally created images. It'll be difficult not to find something you enjoy. Heck, if you only enjoy the wallpaper for a day, it doesn't take any effort to change it.

Here's the list:

1. Social Wallpapering

   I absolutely love this site; it has clear layout and is simple to navigate. This site also has a feature which allows people to vote up the wallpapers they enjoy and vote down the ones they don't. The highest rated wallpaper on their site that I found while writing this was +600; surprisingly, though, the lowest rated wallpaper was only -4, rather than something like -400. There is a large variety of absolutely stunning wallpapers here, and the unique (well, for a desktop site) up-vote/down-vote system works great.

2. Wallpaper Database

   This site was a recent discovery for me; the majority of the wallpapers I have seen were nature-oriented, but all of them were true photographs and not digital renderings. What I appreciate about this site is the (usually) very large selection of resolutions to fit nearly any screen, from a 1080p screen to an iPhone screen.
   

3. DeviantArt

   This site is likely to be one of the more well-known wallpaper galleries. DeviantArt has a thriving community of artists and as a result a near-endless supply of wallpaper. The number of categories this site has for its wallpaper submissions is very diverse; the main point of this site is for its members to share their artistic side, their creativity. There are a lot of categories/sub-categories, so I highly recommend checking out this site. It'd be quite surprising if you could not find something here.

4. Desktop Nexus

   Another intuitively designed, easy to use site. This site was the source of my current wallpaper: a beautiful shot of an Alfa Romeo 8C Competizione. This site does tend to be better for those looking for celebrity wallpapers and car wallpapers.
   Don't think it's limited to just those categories, though; they, too, have quite a variety of cool wallpapers.

5. Interfacelift

   This site is not only a great resource for wallpapers, but also for icons and themes for both Windows and Mac. The wallpaper galleries are simple to navigate; it's easy to look for the most popular wallpapers (by downloads, ratings, comments) as well as by resolution. Having sections for icons and themes, and then to have both Windows and Mac sections makes this a great site for those who have a computer, PC or Mac.

In all honesty, five sites is only a small fraction of all the desktop/wallpaper sites out there. These are the five I enjoy using the most, and share the same important qualities: an intuitive layout, a diverse wallpaper gallery, and a proper selection of resolutions, whether you're on widescreen or fullscreen.

Now that you have the knowledge, go and swap out that uninteresting desktop background of yours into something more intriguing.