From Little Tin Box:
This program gives the illusion of having a transparent desktop.
There are a large collection of photographs on the internet where people have created this transparent illusion. However, creating it is a bit fiddly.
Without this program, you'd have to take a photo, manually work out where your monitor will fit on the image. Crop the picture. Fix the skew. Resize. And if your background contains a pet, its going to be long gone by the time you mess around.
With this program, all you need to do is take three photos.
One of the background, one with the monitor in place and a third after you've fed the two images into the program.
You can also create this effect on any device that allows you to show your own images. The screen detection relies on displaying a colored image on the screen when taking the photo of it in place. Manually put the same image on your other device, tweak the image size to match the device's screen and the program will create it ready to be put on the device.
Best results are with a camera on a tripod, but a camera phone can still give you reasonable results.