Here's a Sokoban ("box pusher") clone called Star Pusher. I've used the graphics from the Planet Cute collection. You'll need Python (2 or 3) and Pygame installed to run it. Just download and unzip the files to the same directory. It comes with 201 levels from David W. Skinner.

Download the source code and graphics.

(UPDATE: I've updated the code so that it works on Linux. It was due to a \r\n newline issue. I've tested the game in Ubuntu and it works.)

The source code is designed to be readable so that new programmers can understand and modify it without much effort.