Duplicates windows live.

Duplicate Windows visually copies any window in Mac OS X live and on-the fly. It’s like display mirroring for individual windows!

Duplicate Windows lets you select from among all window objects known to the operating system. This includes the menu bar, all the UI services icons (the little black icons on the right side of the Menu Bar), and many other things you may not have known were windows. Once you select a window, the new copy changes its size to match the window you’ve asked it to copy. When you resize the new window, it maintains the aspect ratio of the targeted window. Mouse clicks and key presses on the copy are not forwarded to the original.

Duplicate Windows is capable of copying a window that is actively playing video, though not without some visible artifacts.