Kensington Wireless Presenter - works on Mac & PC without any software install. clever design slots the USB wireless transmitter into the handle which locks the unit when not in use to save your battery from inadvertent presses. Add to this a laser pointer and a black-out button for less than £20 and it is a good buy. The latest model includes a 1GB memory stick within the USB wireless transmitter.
The Logitech R800 is my favorite presenter. I used to be a fan of the Kensington above also, but the Logitech has since replaced it. This is perhaps the best green laser presenter out there.