Silverback makes it easy, quick and cheap for everyone to perform guerilla usability tests with no setup and no expense, using hardware already in your Mac.
Moving Sonic the Hedgehog to the right caused the foreground to move past the camera to the left faster than the background, creating a faux-3D view of Green Hill Zone. We can create a similar effect on a web page by fixing semi-transparent background images to different sides of the browser viewport, and at different horizontal percentage positions.