When you’ve several applications open and you want drag a file to a window, for example putting a JPEG into a Pages layout, and the application’s windows are not in view, Leopard has a solution.
Dragging the file to application’s icon in the dock would try to make the application open the file, but if you hold the space bar down while you drag to the icon, it pops the application’s window to the front and now you can drop that file right into place.
There is another way of doing this, and one that can be used in Tiger, and that is to drag the file and then use a ’show all application windows’ exposé shortcut (drag corner or key press) and then drag onto the window and hold there.
But now it’s much easier. Of course.