Doom
The original FPS. Still perfect.
Emulator
DOSBox-X
Quality
Excellent
Notes
Shareware episode freely distributable. Bundled with Velocity.
Included with Velocity
View on Steam →Shareware (Episode 1) is freely distributable — included with Velocity. Full game available on Steam.
The Story
John Carmack's BSP rendering engine made Doom's world feel three-dimensional on hardware that was technically 2D. id Software released the shareware episode for free and sold the full game by mail order — a distribution model that changed how software was sold. The modding tools shipped with the game. Doom is simultaneously a perfect action game, a technical landmark, and the origin point of modern online distribution.
Trivia
Doom ran on every platform imaginable — including a printer, a pregnancy test, and a calculator
Shareware Episode 1 was downloaded 10 million times in the first two years
The BFG stands for 'Big Fragging Gun' in the official game lore