Rogue for Windows
The original 'Roguelike' from 1980!
If you don't know what a roguelike game is, you may want to check the links on the homepage.
In short, it's a dungeon crawling game, where all the dungeons are randomly generated, and the goal is to fight your way to the bottom level, retrieve an artifact, then ascend back to the surface.
Developer: Geoff Dunbar (windows port), open source
Download in the open source Indie Game Installer