Are you getting tired moving the mouse around all day? Are your hands aching? Then it's time for your neck to share the discomfort. HeadMouse is a free software the uses a webcam to detect movements of your head to move the mouse around on the screen. Even though it's designed specifically for people with mobility impairments, I'm sure anybody would have fun using it.
The program has to be first calibrated by turning your head until it correctly detects each movement. After you are done calibrating, you can move the mouse in any direction by turning your head in that direction. You can also reverse the movements the same way as "inverse mouse" behaves in games.
Clicks can be made either by blinking the eyes or opening and closing the mouth.
The webcam should have a resolution of at least 640x480 but most new webcams supports that and even higher. Remember though that not all webcams might work.