Rigid Body Simulation (RBS) is an interesting problem in animation. The space where physically realistic computer-simulated animation could be useful is vast; scientific visualization and computer games are two of the more obvious applications. Unfortunately, the ways in which objects in our world interact, while almost fully understood, are prohibitive computationally in terms of their complexity (hence the creation of strange entities such as the "frictionless table" and the "massless rope" found so often in elementary physics textbooks).
Bit-twiddling can only get one so far; in order to make any sort of real progress in lowering simulation time, large amounts of work must be cut away. Brian Mirtich outlines one such approach in his SIGGRAPH 2000 paper Timewarp Rigid Body Simulation. I have implemented a version of his approach.
Here are some animations of the results: Click on the images to play the quicktime movies.