I have tried to write a simple ideal-gas simulation that demonstrates the Maxwell-Bolzmann distribution of velocities. The "atoms" are circles with different radii and masses, and all collision are perfectly elastic (both energy and momentum are conserved). After few moments, typical Maxwell-Bolzmann distribution is achieved. The ratio between the root-mean-square velocities of the atoms with different masses is very close to the one predicted by the collision theory.

Here is a Youtube video:

http://www.youtube.com/watch?v=YgGik5q1JSA