Works on the old 'conductance of the human body' trick, but updated with USB. You have to have a wire attached to your body in some way for the circuit to complete - the video doesn't always make that clear, or place something (like your body) between 2 contacts.
MAJOR EDIT - Its also a fully functional Arduino. I think that puts it in the 'no brainer' category.
http://www.kickstarter.com/projects/joy ... r-everyone
And before anybody says it, yes you could make this yourself. In fact it is open hardware so they'll even *help* you make it yourself. But its a cool product for a sensible price that could lead to hours of geeky fun - what more do you want