Here is a very handy tip for all of you who like to have a messy and crowded desktop, with hundreds many windows open at the same time.
You are writing an article and referencing at the same time from another source, say the web browser. Now you need to move that browser window in the background, but it really annoys you that every time you do that, you lose the focus of your main window, or even a group of windows. Photoshop, anyone?
Don’t worry, doesn’t need to happen. Simply hold the Command key down, then click on the window in the background and move it.

The window will move in the background, without affecting the harmony of your desktop mess. You can even move it ‘through’ the foreground window, it will just keep going like there’s nothing in its way.