Magic Trackpad, in both OS X and Windows flavors. The download is pretty amusingly huge at 75MB, since it includes videos of all the gestures in action, but at least you'll know exactly how many fingers to use for that new three-finger drag gesture. The bad news? It requires OS X 10.6.4, so you're out of luck if you're still running Leopard and itching to get your swipe on with Apple's latest peripheral.
The new software also updates the MacBook and MacBook Pro multitouch trackpads with inertial scrolling and three-finger drag, which is pretty nice -- we've got it installed on a unibody MBP and it's working as advertised. Hit up Software Update now, or check the source link to score your bits old-school.