This is my first attempt for a CPU based DEM (discreet elements method) simulator ( more info here: http://http.developer.nvidia.com/GPUGems3/gpugems3_ch29.html )
It's based on a Nvidia demo I saw running with the GPU, It was really cool, so I tried to do the same but with the CPU and multithreaded. Of course is not so fast as the GPU version, but it´s not too bad. 100000 particles - 18 fps.
Note that the drawing is not optimized at all, so it´s much slower when I draw spheres. Probably it could be used for a sand simulation or something like that. I made tests at around 1fps with one million particles. I'm now writing a plugin for Houdini.