My latest asset for the GameMaker Marketplace is a Tetris demo. Fully-featured, and configurable, it requires only sound files to be added to complete the project.
It’s meticulously researched, beautifully coded, fully documented, and rigorously tested, and represents approximately 150 developer hours of work, for only $4.99. It’s playable as is, right out of the box. It’s easy to understand the code, easy to configure with simple changes to the code, and modding is encouraged.