Mac quick tip
I discovered this by accident, but my life has been measurably better since.
You probably already know that you can switch apps quickly with Cmd+tab. But if you reach your pinky up a bit more and hit Cmd+~ you can rotate through the windows of the current app. This turns out to be most useful when, say, your email compose window gets behind the main email client window.
What is the equivalent keystroke on Linux? -m