I've been using Backdrop for years, so I was eternally grateful to see it on the Mac App Store, still at the low cost of free. While it hasn't added any bells and whistles (I don't recall the "pixel test" before, but perhaps because I didn't use it) the app does one thing brilliantly: it places a backdrop over your desktop for taking screenshots, doing demos, etc. That's it! You can customize the backdrop color, add an image, or have it float on top of your desktop clutter (my preference) or under it, obscuring only your wallpaper (that pic of nyan cat won't be the butt of boardroom jokes anymore!).
Backdrop is a simple, useful app and it has one more trick: Pixel Testing. Really this is no more than a big red, green, blue, white or black overlay on your screen to check for dead pixels. The colors cycle through a time interval you set, giving you time to look for dead spots where a pixel may be stuck or dead.
If you need something to cover your cluttered desktop while taking lots of screenshots, Backdrop is a one-trick pony that does what it does exceptionally well, for free.