If you want an easy, one key shortcut to Exposé and Show Desktop, go to System Preferences→Keyboard. Under the Keyboard tab, check the box for Use All "F" Keys As Standard Function Keys. After that you can just hit F9 or F10 to activate Exposé and F11 for Show Desktop. Thereafter, however, you will have to hit the Fn key to activate "special" F keys - volume up, down, mute and so forth.