Camera Mouse is a free tool which allows users to control their mouse pointer with head movements.
Launch the program and it immediately displays your webcam feed. Make sure the camera is stable, with your head in the centre of the picture, then click a feature on your face - maybe the tip of your nose. And that's it. Move your head gently and the mouse pointer should move, too.
To click, hover a mouse cursor over an area and wait for (by default) a second. The pointer wobbles a little as it's responding to even the tiniest head movements, so this isn't great for precision tasks, but we managed to click buttons without difficulty, and navigate menus with a little practice.
Enable the double-click option and you're presented with a special "double click" button. Single-click this, and your next single-click counts as a double.
A host of settings allow you to fine-tune camera control, the clicking process and other details.
Conveniently, if this doesn't work at any point, you can disable Camera Mouse with a hotkey, or by moving the mouse and using that instead.