Reader Alex B. sent in this — Fantastic Contraption — a physics flash game where you string together wheels and joints in order to carry an object toward a goal on the screen. It's nice nonviolent trial-and-error fun, indulging both the competitive urge to be as efficient as possible, and the creative impulse to be as outlandish as you can get away with.
The opening tutorial is pretty helpful, and many of the opening levels are easy to beat in minutes or seconds. As for the general appeal of flash games, I think XKCD summed it up best.
Fantastic Contraption [Site, thanks Alex B]