Someone named Cory, who seems to be both a Nintendo and a GoNintendo.com fan, has created the coolest wallpapers I've seen in some time.
I think this one is my favorite, though they're all awesome:
Cory also created wallpapers inspired by such classics as Kirby's Adventure, The Legend of Zelda, Metroid, Pokemon and Super Mario Bros. 1, 2 and 3. You can find (and download) all of them here.
You also may want to check out GoNintendo's original post about the wallpapers, which includes a few comments from Cory himself.