One of the first things I show my mates who have recently made the move to Mac, is Exposé Active Screen Corners (Hot Corners).
Hot Corners allow you to trigger an action by using a the simple gesture of pointing the cursor into one corner of the screen.
Configured under System Preferences, you can program what each corner will do:
It’s so naturally useful that I’m surprised I haven’t seen it in Windoz yet. It is available in Ubuntu Linux using Compiz.
If you own a recent MacBook, you can also configure the “Four Finger” mouse gesture to trigger the ‘All Windows’ Exposé action from the trackpad.
Here is my preferred Exposé setup: