Physion is described as '2D physics simulator that runs entirely in the browser, with nothing to download or install. You draw rigid bodies directly on the canvas (circles, rectangles, capsules, polygons, regular polygons, gears, polylines, spirals and text bodies), connect them with' and is a Sandbox Game in the games category. There are four games similar to Physion for a variety of platforms, including Mac, Windows, Linux, Web-based and iPad apps. The best Physion alternative is Algodoo. It's not free, so if you're looking for a free alternative, you could try Algodoo or SimPhy.