Zandronum

Zandronum is a classic 90s first person shooter engine, modified to play both online with other players or alone in single player. Zandronum comes with community made game assets, called Freedoom installed, so you don't need to own any classic 90s FPS games to play. If you want to play one of the official Doom games with Zandronum instead, copy your WAD files to a folder named Zandronum in your Downloads folder. Zandronum is open source software under the Sleepycat License and GNU GPLv2 Source code can be found here: https://github.com/Vritty/zandronum