Tetris
This was nothing more than a mere programming exercise, and fun nonetheless. Written in C++ in 2006 and used Direct3D 8, the SDK of which is not even distributed by Microsoft anymore. Good luck compiling this.
Screenshot of Tetris running in Windows XP.
The compiled Tetris game works on a variety of Windows environments and, cool enough, without any modification or foresight, in Wine on Linux. (Where is your OpenGL god now?) Feel free to try it out and see how terribly unoptimized it is, if you trust a stranger on the Internet enough to to run an executable on your computer.
The source code is also publicly released under the BSD license... like anybody is going to use this code in any project.