Aero Peek is a tool that will help you do this. For example, if you want to have Windows Seven's Show Desktop button, simply install Aero Peek.
Once it is installed, Aero Peek will appear as an icon on your tool bar. If you click on it, you will be able to see those windows that are open and they will become completely translucent, so that you can see your desktop.
Click on Aero Peek again and the translucent windows will become opaque like that previously were.