The green button in QuickTime!
The green button in the upper left corner maximizes or resizes the window to the optimal size. I really don't like the way it works in web browsers and most other applications, but it is completely different in QuickTime. It will actually resize the window to the "Actual Size". What is even better is that unlike other applications, it remembers the size the window was before resizing and clicking it again will resize it back.