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Fantastic Contraption

From the GeekDad blog:

Fantastic Contraption is a fun physics puzzle game where you create assorted machines to reach a goal.   You can play 20 levels for free at their site.  If spending money is your thing, $10 will give you access to a level editor as well as the ability to try out other players saved work.  Good stuff!

Update: Greg wrote in to mention another cool physics-based game called Magic Pen.  Thanks for the tip!