The new site Social wallpapering contains lots of beautiful wallpapers for dual monitors.
One man, two monitors
On my desk I have three screens, synchronized to form a single desktop. I can drag items from one screen to the next. Once you have that large display area, you'll never go back, because it has a direct impact on productivity.
The multiple monitor revolution is rolling! Some companies have started manufacturing double-screen monitors, some even triple- or more. They work like separate monitors but looks better.
One company is Digital Tigers, which have their Zenview monitors with two or three or up to eight screens in one unit that is curved in the right ergonomic way so that each screen is in the same distance from the user.
Windows cannot in itself show two different wallpapers on two monitors, but it will span the wallpaper over two or more monitors if the wallpaper is wide enough.
If you have e.g. two monitors at 1280*1024, you need a wallpaper with the dimensions 2560*1024 - or at least something close (assuming your monitors are next to each other, of course)
I found very beautiful wallpapers with this width at interfacelift.com.
Other sites with wallpapers for dual or triple monitors is dualscreenwallpaper.com, 9x media and XXL Wallpapers and more.