Digital Blasphemy was one of the first desktop background sites I came across, and it's still one of my favorites. Most of his excellent work is subscription only, but there is a rotating free gallery where I've found some great stuff over the years, and there's also a large user gallery showcasing the work of other talented artists.
Ferenc J. Haraszti was one of the geniuses I turned up in Digital Blasphemy's user gallery. I love his detailed scenes of landscapes and urban European locations, like old shops, charming street scenes, and statuesque cathedrals. His work has a rich, rough texture, almost like a painting or pastel, that is a refreshing change from the slick, seamless, inorganic look that characterizes most digital art.
Visual Paradox has several large galleries, divided into themes such as real-world, creatures, sci-fi and fantasy, and objects. To be honest, a lot of them are duds, but scattered throughout are a few outstanding gems that are really worthwhile.
Plasma Design is another subscription-based site, but there are a fair number of lovely free images mixed in with the member-only ones. Very nice, smooth land- and waterscapes, divided into sunrise, day, sunset, and night galleries.
Back when I first came across it, Shifted Reality was a nice collection of some quite good landscapes. Checking back now, I see the artist's talent has really developed, and the newer works are simply stunning. The scenes have become broader and more detailed, with a lot of nicely textured aerial shots of alien worlds along with some closer views. A lot of the images remind me of conceptual renderings of Myst ages.