Multi-Monitor Wallpaper Stitcher
Take multiple wallpapers and create one large bitmap for a Windows background that spans multiple monitors. Up to 36 monitors supported (like I'd actually be able to test that). Monitor centric selections that allow you to fit to the width or height of each monitor or simply freestyle drag and zoom within each monitor.
Quick Feature list:
The only real issue I've found during testing is that occasionally you'll get a pixel thick line on one edge of the wallpaper. For now, just make sure to change the background color of the monitor to something close to what's in the wallpaper. I'll be working hard on fixing this as well as improving the software in general.
This program requires .NET version 4.0 installed before it can run. If you're running Windows 7, then you should be okay. Otherwise, you'll want to head over to the .NET installer page.