Silas named this activity. I think he was inspired by the "plunk" made when the marbles crashed into one another in the cup -- a most satisfying sound.
This is just a container pulled from our recycling bin with a short length (about 9") of 3/4" vinyl tubing pushed through a hole cut in the lid. Add a bowl of marbles and you have a simple, introductory marble run.
Silas was really jazzed about this one. He had me empty out the container and reset several times and he kept at it for about 30 minutes. He turned the cup so that the entrance of the tube was facing different directions and he experimented with using both his left and his right hand.
This is great for hand-eye coordination, fine motor skill development, and visual tracking (a pre-reading skill). And it's fun.