I'm a big advocate of making your computer your own. While there are a ton of applications out there for this purpose (and they can be quite cool), most of them involve tweaking and time-consuming work. The easiest way to add some life to your desktop is with an attractive wallpaper image.
I've got a couple of favourite sites that I check regularly.
interfacelift.com features daily high-resolution pictures of mostly landscapes, seascapes, and nature, although they post some macro shots and abstract stuff on occasion. And it's all available in a wide range of resolutions, wide-screen and standard.
For computer-rendered images, I usually check out
digitalblasphemy.com and
visualparadox.com. Both have high-quality pictures of a whole range of subjects, such as nature, people, space, abstracts, and fractals. It seems Digital Blasphemy is no longer completely free, but the artist still hosts a selection of his work in a free gallery.
Most people I think have heard of
deviantART, an excellent website full of uploaded artwork of all sorts. They have a decent wallpaper section if you're into hand-drawn art.