you can program 'hot corners' on assign a particular key to do that. here are the steps for each
Hot Corners: You can go into one corner of the screen and the computer will perform the assigned action. 1. go into System Prefs 2. Click 'Exposé and Spaces' 3. Click the scroll menu of the corner you want to activate and scroll down to 'Desktop' 4. Go all the way into that particular corner and the desktop should appear. Go back into the corner to show all your windows again.
Assigning a Key: 1. On the same pane as Hot Corners, click the scroll menu next to "Show Desktop" and click the key you want to assign to show the desktop when pressed. Make sure that this key has no other function, such as volume or brightness.
I know that's a lot of info, but it's worth writing it 🙂
-MD55