The arrival of Lego mindstorms kicked off a wave of Lego innovation, but not all of that used official Lego mindstorms kits - some where just motors strapped onto curious contraptions, some of the most interesting being the Rube Goldberg devices, kinetic sculptures, rolling ball machines and similar.
Lego Domino laying contraption:
Early Lego marble machine:
Innovations like this have led to responses with pure Lego, the culmination being the Great Ball Contraption - GBC - This video has trains, robots, and many ball moving machines:
The GBC is many contraptions made by many Lego clubs and fans, unified by a common interface and brought together at Lego gatherings where many are joined end to end to make a circuit. These lead to videos of smaller collections like this one, filmed in a way that makes it clear it has become obsession: