The MacBook Air and newest MacBook Pros include MultiTouch-capable trackpads that let you rotate, resize, and gesture your way through many common tasks. For instance, you can increase and decrease font size in Safari by making a pinching gesture with two fingers on the trackpad.
You can use three fingers swiped left or right to go forward or back in your browser history. I very recently discovered that the three-finger trick works in Mail on my MacBook Air: three fingers to the left gets you to the previous message, and three fingers swiped to the right gets you to the next message. Pretty cool!
Tell us about any MultiTouch tricks you’ve discovered in the comments below!