I happen to love Marble Runs as much as I do my Legos. Here are a series of Block-N-Roll Marble Runs I have made. What is so cool about Block-N-Roll is that they attach to any Duplo Lego pieces so they are perfect for building marble runs around your lego world. The pieces not only attach to Duplos but they attach to each other so you can have long runs in between your support beams.
I wish I could get ahold of the people at Block-N-Roll, I so many ideas for them on new products. They also really need to get a new salesforce because you can't find their product anywhere and its such a great product you should see it everywhere Legos are sold.
The first creation I call the Waterfall Timer. Sorry for the quality it was shot on my iphone.
This one I call the Zig Zag.
This one I called the Marble Bobsled.