Chess Clock for >2
I play at a soccer club where they have no real goalies, so the field players have to take turns in the goal. I always have the feeling that some people try to dodge their responsibility while others spend a fairly long time in the goal, but there is no real proof.
The solution would be to have a chess clock type of device, only with five or more buttons. If you're in the goal, you press your button, and as the game proceeds you can easily tell who has spent the least time in the goal, and send him to fulfill his duty.
I am sure there would be countless other applications for this grand idea!
Reward: A soccer chess clock for free (and maybe a backup if the original gets destroyed by the ball)