Algodoo

A simple thyme thruster. There have been some designs like this, but I haven't seen any with as few parts.
Basically, there are two boxes. The top collides with A and the bottom with B. The top one gives the ball a low mass, and the bottom box makes the ball very heavy.
When the box touches the top, it gets very light, and the lower box pushes it up. The next frame, the ball is made heavy, and again collides with the top box, so it goes down. The cycle repeats.
The scripted masses can be changed easily to change power. 2 versions are included. In one, the ball is held in the contraption by a box. In the other, a very strong spring holds it in place. I think that the spring version is slightly weaker, but more reliable.