Algodoo

I tried to make some new kind of magnetic engine but instead came with that
Those are very unstable to be engines and have random behaviour - they make those sudden bursts and can change direction of flight randomly, see for yourself. Maybe that idea can be used to create some kind of a random moving enemy in a Phun game.

S - stabilize (destroys outer circles and makes outbursts to appear less often)
T and Y - free float
G and H - let particles loose (blue particles have negative attraction, and white have positive, so they'll swarm around objects you put in the scene)