WindowSpy is a new Unity AppIndicator that allows displaying a small live (well, almost) preview of a window on another workspace.
Thee tool is useful if you want to keep an eye on a window on another workspace, waiting for something to finish. For example, you can use it to see the progress of something that’s running in a virtual machine. WindowSpy is not exactly useful if you want to use it to watch a video in the preview, because the preview is updated every 3 seconds.