Remember that boolean operator tutorial disguised as a "build your own Tetris" article I posted here a couple of years ago? It contained a lot of text, some illustrations, and even a very bare-bone game if you stitched the code snippets together. Well, I returned to it and wrote (yet another QuickBASIC) Tetris clone.
It has some neat modding features and supports three game modes: "treadmill" (makes the well shift horizontally every turn,) "junkyard" (fills the bottom of the well with some junk,) and "high tide" (increases the size of the pile every few turns.) Modes can be combined together if you feel like trying something spicier than the standard vanilla gameplay.
There's also a high-score table, customizable keyboard controls, PC speaker sound or music, two types of blocks (standard and kid mode,) and three different backgrounds to choose from (more can be added.) Everything is sourced as usual.