This is based off an idea Darren Cullis gave me a year ago but with a little tweak by me.
A cube emits some spheres that have normal "Simulate Rigid Body Dynamics" applied. They also have a custom force where by each particle tries to fight its way back to where it was emitted.
Rendered in mental ray using photometric lights.
P.s. there's a jump near the beginning due to this not being cached and the simulation changing slightly.