Isn't it annoying when you click a link in an email and Safari pops up in front when you want to continue reading and check the web page later? Mac OS X offers a way to open and manipulate windows without bringing them to the front, by holding down command whilst doing things.

For example, command-click a link in an email. Safari opens up in the background, without disturbing your reading, allowing you to go to the page later.

This also extends to moving, closing and resizing windows. If you can see a Finder window behind your iTunes library for example, command-clicking on close, minimize or holding command whilst moving or resizing performs the action without switching to the finder, instead staying within iTunes.