This is a project done at my local hackerspace, we were a few working together.
The brief was to build a ball track which randomly would select an occasional player to win, where scissors cut a ribbon and travel suitcases fall down. (The prize is an airticket) We only had two weeks. This project was mostly woodwork and bending lots and lots of copperwire. My major contribution is the helix lifting the balls, and the whole base support (it had to go through a small door in the exhibition window, thus had to be dis- and re-assemblable)
I also did the Arduino part. A simple serial interface (master pc sends request, Arduino replies, all single char) would launch a ball. The Arduino had some timeout loops in case the ball got stuck- It starts and stops the feeder motor and transport helix, and two sensor determine if the ball came out in the kooser bin or released the scissors. A couple of IR distance sensors allowed people outside to influence the track for the final Galton board randomizer.
The "master pc" was actually also just a slave/gateway pc thet taled to another server pc somewhere which did some HTTP stuff to a SMS-Message service - the upshot being that the persone outsiode on the street sends an SMS, one ball does a run and he gets a SMS back in case of a win.
So in terms of "Arduino smarts" a very simple project - small but vital :-) On to the Video!! http://www.youtube.com/watch?v=MIMg_wMWfLQ