Backdrop 1.5

Backdrop is a simple utility to fill your screen with a giant blank window. Fill the window with a solid color or a custom image, your choice.

This is useful for:
Screenshots - take screenshots of one or many windows without having to clean up your desktop
One thing at a time - Use Backdrop in between applications to let you focus on a few windows at a time. Want just one window of an app? Switch to Backdrop, then use Expose to pick out the window.
Web design - create a full size image with outlines of more common screen sizes to help to check how your pages will look on different monitors.