Here’s something that I’ve kept to myself for a while:  To move a window in the background without bringing it to the front, ⌘-click its chrome and drag.

But wait, there’s more!

It’s not just the chrome that reacts this way.  It’s any element of the background window.  Select text, refresh a page, close the window, pause a movie (iTunes’ buttons are natively able to be used without bringing the application forward); anything.

This is one of the many reasons I have my scroll-wheel button set to ⌘-click.  I also use it to open links in new tabs and deselect icons and list items (it bothers me when an email item or desktop icon is left selected;; I keep the bodies beneath the floorboard, etc.)