Whether you're taking screenshots/making a screencast and you want a bare background, or you prefer to work with as spare a workspace as possible but you can't bring yourself to keep your desktop clean, Backdrop is a great tool. Just start it up and it automatically places a layer over your desktop and all your open apps. Color of your background is your choice.
Fills your screen with a color or image
Take screenshots with a clean background
Supports multiple monitors
Use Backdrop in between applications to allow you to focus on one thing at a time
Copy Backdrop to a desired location on your hard drive (may I suggest the Applications folder). Open Backdrop from your hard drive to enjoy.
Note: Not copying to your hard drive and running from a disk image may cause Backdrop to crash.
I have it installed on my Mac and it works perfectly!
Mac World gives Backdrop a 4 Mouse rating!