Marble machine tricks
Looking for some mechanical puzzles (which I love), I ended up on the DMOZ Puzzles page. And it has a sub-section for Rube Goldberg machines (see this previous entry for an example).
That's how I found this Matthias's marble machines page which explains a couple simple but neat principles that he used for the marble machine 1 and 2: a simple marble flip-flop and two marble pumps to brings the marbles back at the top.
It's quite fun to try and figure out other ways to brings the marbles back at the top. Can you come up with some non-trivial ones of your own?
http://www.kugelbahn.ch/ has links to many other kinetic art websites, with an endless supply of sculptures, automaton and games.