Physics engines simulate physical processes in a more or less realistic way. Simulations that come up with results that are accurate enough for research purposes aren’t available in real-time yet, but faster computers and specialized hardware for just this purpose should make that possible in the near future.
These two flash based ones are two-dimensional and work in real-time, but results are certainly realistic enough for what they are supposed to do:
This one is especially cool, draw some shapes with your mouse and watch them fall and interact with each other. You can also errect simple structures on the ground and combine them to gte larger buildings.
I simply can’t wait for the three-dimensional versions that are sure to follow.