Do you use multiple monitors?
Do they have different resolutions?
Have you ever wanted to display different wallpaper images on each monitor, or to have the same image displayed appropriately (without cropping or stretching and the like) on both monitors?
Well, welcome to DisplayFusion (http://www.binaryfortress.com/displayfusion/)!
DisplayFusion is a free software utility that lets you quickly and easily manage multiple monitors. It does more than just give you control over the desktop wallpaper (although this is all I'm using it for today) - there's also a nice list of features related to managing window position across monitors with global hotkey support.
And did I mention it's free?
So if you answered yes to any of the questions above, you should download DisplayFusion and give it a try.