The Lego community, especially the AFOLs have created a system for interconnected ball passing modules. Each of the modules simply has to pass balls from an in hopper, and output into the next machine. What is done in the middle is entirely up to the designer of the machine. It is strictly only Lego parts though.
These make for a colourful, lively display - with each module imaginatively made, showing great engineering, sculpting or architectural ideas. When Lego groups meet up, these modules are brought together, with the modules creating a circuit - this results in a huge collective Rube-Goldberg machine, and to participate, a punter brings along a module of their own on the next visit.
You will see all the Lego themes represented - this is not just the realm of Technic and Mindstorms. The systems may be built on a single motor, or many, or even with a number of pneumatic elements. Some elevate the balls to a height, so gravity and clever widgets give the ball interesting things to do all the way down to the next module. Some are like clever conveyors, some are like complicated automatons.
The initial specifications were drafted during a discussion on Lugnet.