I really don't know much (yet) about Sketchy Physics. It's a plug-in for SketchUp, and according to the YouTube videos I've seen, enables you to do incredibly cool things. With the release of dynamic components in Version 7, you can do similar things with moving parts, but only Sketchy Physics seems to follow the laws of Newton.
For example, check out this video, which is about a year old:
When I get some free time (what's that?) I'd love to dive into this a little deeper, and maybe write some tutorials about how to start with some simple models. This seems like something every physics teacher should know about.
Anyone can design anything in 3D! www.3dvinci.net