Square in a Sandbox
Box2d Demo

arrow keys to move, space to jump
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health 100        score 0

Knock stuff over, collect coins, feel the pain of big falls and crushing accidents.

Box2d is the physics engine made famous by Angry Birds.

The project is open source, and free to use (zlib license), so you might want to check it out. The engine shines when simulating many colliding free-moving bodies. Try manipulating the environment of this demo to see what I mean. If you are considering making a 2d game or other simulation requiring complex 2d physics, you might do yourself a favor to use Box2d and avoid reinventing the wheel.

javaScript Box2d download link