Duplicate Windows allows for real-time graphical duplication of any window in Mac OS X.
DW was developed for the purposes of duplicating windows across multiple monitors during demos and presentations. This allows the presenter to duplicate a program they're interacting with on their laptop screen onto a projected screen, while keeping private notes only visible to the presenter. In that usage, it's like mirroring displays for individual windows or "presenter notes" for any application.
DW is also extremely useful for watching web video. By duplicating a source window, you can arbitrarily resize it, and keep it small and on top of other applications. It can similarly be used to monitor the state of applications you are not currently interacting with.