There is a definite lack of decent dual-monitor wallpapers out there. This is my attempt to slightly fill that void.
My post about dual-monitor techniques can help you set up wallpapers for both screens on Windows systems.
If you run on a Mac, your system has far better multi-monitor support, so should be no problem.
Description: Loosely based on Aurora Borealis [edit: Link now unavailable]. The tutorial he linked to was down, so I improvised and made this in about five minutes. The effect was pretty much what I wanted.
Description: Was practicing for different underwater scene techniques for the scenes in webcomic I used to work on, and thought it would be a cool effect to have these rays emanating from that space between monitors.
This wallpaper uses some of the techniques outlined in Vlad's underwater tutorial.
Other quality dual-monitor wallpaper sites: