If you use dual monitors and have noticed that desktop wallpaper hasn't quite caught up to you yet, have a look at this site. I've also added another site that has dual monitor desktop wallpapers and many triple monitor desktop wallpapers.
Smashing Magazine once again comes through with a list of 70 dual screen desktop wallpaper, collected from various artists and designers around the world.
Mandolux is a site that has wonderful (and sometimes a bit baffling) wallpaper for dual screen displays, but what sets them apart is the quantity of triple screen desktop wallpapers available.
Many of the dual screen wallpapers are also available as triple screen desktop wallpapers, and that adds up to bunches of both kinds of desktop wallpaper.
Smashing Magazine's 70 Beautiful Dual Screen Desktop Wallpapers
Mandolux Dual and Triple Screen Desktop Wallpapers [Currently not available]