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Doom
Doom is a legendary video game released in 1993, perhaps the most famous video game of all time, the game that popularized the first person shooter genre and shocked by its at the time extremely advanced 3Dish graphics. It was made by Id Software, most notably by John Carmack (graphics + engine programmer) and John Romero (tool programmer + level designer). Doom is sadly proprietary, it was originally distributed as shareware (a free "demo" was available for playing and sharing with the option to buy a full version). However the game engine was later (1999) released as free (as in freedom) software under GPL which gave rise to many source ports. The assets remain non-free but a completely free alternative is offered by the Freedoom project that has created free as in freedom asset replacements for the game. Anarch is an official LRS game inspired by Doom, completely in the public domain.
{ Great books about Doom I can recommend: Masters of Doom (about the development) and Game Engine Black Book: Doom (details about the engine internals). ~drummyfish }
Partially thanks to the free release of the engine and its relatively suckless design (C language, software rendering, ...), Doom has been ported, both officially and unofficially, to a great number of platforms (e.g. Gameboy Advance, PS1, even SNES) and has become a kind of de facto standard benchmark for computer platforms -- you will often hear the phrase: "but does it run Doom?" Porting a Doom to any platform has become kind of a meme, someone allegedly even ported it to a pregnancy test (though it didn't actually run on the test, it was really just a display). { Still Anarch may be even more portable than Doom :) ~drummyfish }
The Doom engine was revolutionary and advanced (not only but especially) video game graphics by a great leap, considering its predecessor Wolf3D was really primitive in comparison (Doom basically set the direction for future trends in games such as driving the development of more and more powerful GPUs in a race for more and more impressive visuals). Doom used a technique called BSP rendering that was able to render realtime 3D views of textured environments with distance fog and enemies and items represented by 2D billboards ("sprites"). No GPU acceleration was used, graphics was rendered purely with CPU (so called software rendering, GPU rendering would come with Doom's successor Quake). This had its limitations, for example the camera could not tilt up and down and the levels could not have rooms above other rooms. For this reason some call Doom "pseudo 3D" or 2.5D rather than "true 3D". Nevertheless, though with limitations, Doom did present 3D views and internally it did work with 3D coordinates (for example the player or projectiles have 2D position plus height coordinate), despite some dumb YouTube videos saying otherwise. For this reason we prefer to call Doom a primitive 3D engine, but 3D nonetheless. However Doom was not just a game with good graphics, it had extremely good gameplay, legendary music and art style and introduced the revolutionary deathmatch multiplayer, as well as a HUGE modding and mapping community. It was a success in every way -- arguably no other game has since achieved a greater revolution than Doom.
However Doom's success wasn't just due to its graphics, it excelled overall in all areas -- it had brilliant fast and gory gameplay, amazing level design (by John Romero), excellent soundtrack and even a revolutionary multiplayer. The game's backstory was simple and didn't stand in the way of gameplay, it's basically about a tough marine (so called Doomguy) on a Mars military base slaying hordes of demons from hell, all in a rock/metal style with a lot of gore and over-the-top violence (chain saws n stuff).
Doom source code is written in C89 and is about 36000 lines of code long. The original system requirements needed roughly a 30 MHz CPU and 4 MB RAM as a minimum. It had 27 levels (9 of which were shareware), 8 weapons and 10 enemy types.
The game only used fixed point, no float!
Doom also has a deterministic FPS-independent physics which allows for efficient recording of demos of its gameplay and creating tool assisted speedruns, i.e. the time step of game simulation is fixed (35 tics per second). Such demos can be played back in high quality while being minuscule in size and help us in many other ways, for example for verifying validity of speedruns. This is very nice and serves as an example of a well written engine (unlike later engines from the same creators, e.g. those of Quake games which lacked this feature).
LOL someone created a Doom system monitor for Unix systems called psDooM where the monsters in game are the operating system processes and killing the monsters kills the processes.