What are some good free physics based games?

Try the demo versions of Crayon Physics, World of Goo, and And Yet It Moves.

Uh...Garry's Mod?

Stair Dismount & Truck Dismount?

The Powder Game:
http://dan-ball.jp/en/javagame/dust/

You basically just toy with different elements and ingredients, and everything reacts differently. It's a little fun for a while.

http://www.gamesforwork.com/games/play-11030-Gravity_Hook-Flash_Game

Pong.

Rock is a good one. Go outside, pick up a rock and see how many windows you can break before the cops come. Bonus point for each cop car that shows up.