WARNING addictive Flash game
I had plans for today, but then I found a link to Fantastic Contraption, and before I knew it I had wasted the whole day. So be warned.
It's a 2-d physics engine puzzle game. Each level has a pink ball that you're trying to get into a goal area; there are five very simple pieces which you can use in unlimited quantities to get it there. (Three kinds of wheels, two kinds of sticks). Everything behaves according to mostly realistic physics, and half the fun is watching your plans go awry as objects tip over, collide, get flung, slide off of edges, roll away, etc.
Unlike other physics-engine puzzle games (yes, this is a genre now), Fantastic Contraption doesn't limit your piece supply or score you based on the complexity of your solution, and levels have no single right answer. So your creativity can run wild. It's up to you whether you want to find a brilliantly elegant solution using a single falling stick positioned just so, or whether you want to use 300 pieces making a giant walking spider bot with back-mounted trebuchet to deliver the pink ball in style. There seems to be quite an active YouTube community posting videos of their proudest achievements.