Windows 7 only: Freeware utility Windows 7 Folder Background Changer adds a background image to Windows Explorer, and even allows you to customise each folder with a separate picture.
Once you've downloaded and extracted the no-installation-required application, you'll probably find that you need to right-click and choose "Open as Administrator" the first time you launch the application, but otherwise it's fairly straightforward to use — just choose a folder to customise, pick the background image and the other settings, and then close and re-open the Windows Explorer window to see the changes.
It's not really going to help you get more work done, but if staring at a white background is getting old, it might be nice to have a better looking workspace, at least. Windows 7 Folder Background Changer is a free download, works on Windows 7 only.
Windows 7 Folder Background Changer [The Windows Club]