Version 1.2 [Windows, OS X, Linux] (Requires Adobe AIR)
As the name suggests, this program is designed to assist in removing backgrounds. You give it an image as the background, and any number of images on that background (in the same position relative to the image), and it should remove it. What it actually does is check for identical pixels in the same locations and removes them, so it only works for pixel-perfect images. Therefore is it designed for removing backgrounds from things like games screenshots, rather than photos.
Alternatively a colour can be chosen and all pixels of that colour will be removed.
All versions prior to 1.2 are Windows only. All versions require Adobe AIR.