http://FantasticContraption.com/
Engineer grassroots solutions to a variety of tasks (well, OK, a variety of "move this payload from here to there" tasks). You get a little kit of parts to build your contraption, and an environment in needs to work in.
It's highly addicting! Give it a shot. Unfortunately, I think their server is getting stressed, but stick with it, you'll get to play if there is a problem.
Create an account (free) and you can save your designs and show them to others - that's half the fun.
One thing they don't tell you in the tutorial - in the ones were the payload starts inside a work box, you can MOVE the payload as you wish inside the box while constructing your machine. That makes a few levels a bit easier.
I'll post a few of my contraptions in a bit. Have fun (assuming it's working)!
