The Fantastic Contraption
This is one hell of a great little game, as most physics games I’ve seen have tended to be. The tutorial’s entirely thorough so I won’t bother describing how to go about it, but the objective is to get the small red object into the big red area. It gives a lot of room for creativity and flexibility, and you can even do awesome things like send the URLs for interesting solutions to other people. For the very controversially-named (and massively flukey) one above see here, although sadly I didn’t save my many tank-based ones, as tanks are awesome. No great loss, really.
If you do come up with interesting ones stick them in comments, yo