Doom engine

GZDoom

Another title that's enjoyed remarkable longevity is Doom. Doom is about as far removed from SimCity (see above) as you could imagine (unless you include the giant lizard attack as one of SimCity 5's disaster scenarios), but it created a genre in much the same way. The first-person shooter (FPS) genre that Doom created is perhaps the most popular of all, with the latest release featuring hundreds of people on the same map at the same time, and graphics and physics that are sometimes indiscernible from reality. Yet the immediacy and relative simplicity of the original has kept it popular – popular enough that there are several open source implementations of its famous game engine, which itself was generously released for free in 1997. GZDoom is one of these, and it's actually a port of an older engine, now discontinued, called ZDoom.

The main advantages any of these recent engines will have are that they'll run on modern hardware and often use as much graphics acceleration as you can throw at them. GZDoom includes advanced rendering for both OpenGL and software-only platforms, which means you get advanced features like colored lighting, 3D floors, portals that connect separate areas of a map, and, of course, higher resolutions for both the display and the textures used within the maps. To play the original Doom with any new engine, you will need the original WAD files. As these have never been released under an open source license, you will need to extract them from your own personal copy of Doom. However, WAD files have been created and released under an open source license, with one of the best being "Freedom," a single-player game split into two parts and consisting of more than 60 different levels!

Project Website

https://zdoom.org/

Likely using fewer polygons in the whole game than a single gun model in a modern FPS, Doom is still immensely satisfying.