You know how when you were a kid, you could play with legos and imagine
them to do all sorts of things? You ever play Lemmings -- you know,
that old school puzzle computer game where you had to build little
things so the lemmings would reach their goal instead of dying horrible
deaths? Fantastic Contraption is kind of a mix of legos and lemmings,
with a bunch of cool physics modeling in there as well.
Basic idea is to get some blocks into the goal, and to build any sort
of machine using rolling balls and sticks.
http://fantasticcontraption.com (do not click unless you're willing to
lose a few hours of your life immediately)
Hat tip to Adam R