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

Prior to 3.1.x the dat subdirectory didn't exist yet but data.base was present somewhere, possibly in the src subdirectory instead. I don't know whether it goes as far back as hack 1.x or even nethack 2.x. The source code for a bunch of old versions is present at https://nethackwiki.com/ but I'm not sure whether the data files are all there too. The source code for the most recent released version (3.6.7) is... Source: about 1 year ago

I am not sure what extra benefits the Steam version has, but the game is available for free either as download on nethack.org, or to play online at something like alt.org/nethack or hardfought.org. Source: about 1 year ago

You don't even need two people to play D&D, Nethack is great! Source: about 1 year ago

I've always been fond of the last quote on nethack.org. Source: about 1 year ago

It was discussed here last fall, when the home page for nethack.org was updated to mention it. The blurb includes a few other tiny screen shots and says the exhibit is scheduled to be open until mid-July. Source: over 1 year ago