We've seen some pretty awesome Rube Goldberg machines in the past, but if you were too lazy or don't have enough room to build one yourself, Frigits may be the solution. This is a set of miniature contraptions that allow you to set up a Rube Goldberg-style machine on anything capable of holding magnets. Impress your friends at the office, or just set one up on the refrigerator. Regardless, it is a easier and painless solution to filling that Rube Goldberg fix. The complete Frigits Deluxe set goes for $30.
Now, as I recall from seeing a few Rube Goldberg cartoons, there was always some simple need being satisfied by operating the contraptions — opening a window, making toast, etc. That's what I think is missing from these sets... some kind of widget or gizmo that does something at the end — turn on a light, activate some kind of motor that returns the ball to the top. Not that it has to be useful — just something at the end before the ball comes to a stop.
In the old sesame street films where the little ball ran down tracks, it would cause little flags bearing numbers to wave around and spin attractively. That might be good, too.