There seem to be quite a few companies in the business of physics simulation these days.
Graham Fyffe pointed out this one to me: Algoryx focuses on 3D multi-physics simulations. They are also responsible for the Phun demo I posted a while ago, that has also been floating around YouTube.
Another company that does rigid-body physics simulations is CMLabs.
Just to get that right – Algory might be responsible for Phun now but they hired the guy who wrote fun after he wrote his first phun versions.