Since I've never gotten around to doing it, yesderday I decided to implement a light and fast tetris-clone in C and SDL. It follows the official tetris guidelines as close as possible, as described here. The source code is written in such a way that it mimics the official guidelines naming conventions, and tries to be as clear as possible. Below is a gameplay demo:
Current plans to extend this implementation are to add "wall kicks", a score and line counter display, an ncurses renderer, and networked play for at least two players. Other modes could also be added on quite easily at some point later in time.
If you're interested in playing, you can download the source tarball: hidamari.tar.gz, or pull the latest version with git: (Unavailable at this time).