Push It

I promise that the link you click on will be ad-free, and that the game is really 100% free. Push It is a Sokoban clone. The gameplay involves moving around a little warehouse map, and pushing big crates into squares. It starts off very easy, and gets progressively difficult as you go along. It has 30 levels, and you can access all of them without any passwords to remember. The level editor is pretty useless, btw.

If you really like it and want something bigger and better, Wonderland is a simular game.