Cube Engine 1 and 2 touted as the smallest (in terms of file size) FPS Multiplayer game. It's also free. Unknown if the community is still booming as I have not played it in a long time. Eh while we're at it there's also Warsow but this one I know the community isn't as great as before. Source: about 1 year ago

I'd you want something a little more advanced but straight forward I highly suggest Warsow. It's a free first person arena shooter that plays very much like Quake. Source: about 2 years ago

Warsow is also quite good and very close to late 90's shooter's idea of multiplayer https://warsow.net/. - Source: Hacker News / about 3 years ago

Playing games is part of who we are as humans. Ditto for making games. From Nine Men Morris to Shattered Pixel Dungeon, Mindustry, and Warsow, people have been making and playing games for free. We don't need a games industry in order to have games. Source: about 3 years ago

GZDoom for Doom and many other games running on the same engine. Source: over 1 year ago

For modern source port check GZDoom https://zdoom.org/index or Chocolate Doom (for higher framerates and stuff) - https://www.chocolate-doom.org/wiki/index.php/Chocolate_Doom. Source: about 2 years ago

Also, if you want to make a shooter like those, you don't really want the original Build engine. What you want is a modern open source implementation of those engines. For Duke-like games you can try https://www.eduke32.com/ (Ion Fury was made on this) , for Doom try https://zdoom.org/index or https://dengine.net/. Source: over 2 years ago

I don't know, but if you want a retro game you might as well use a retro-engine. Ion Fury (2019) used the EDuke32 engine and there are also modern versions of the original Doom engine, like GZDoom - with modern games like Brutal Doom or SIGIL (2019). Source: over 2 years ago

Because zDoom and GTA anniversary releases are a thing. Source: over 2 years ago