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

Shattered Pixel Dungeon - This is a variant of the original Pixel Dungeon. This version is better balanced and has more content. Available for PC, tablets and phones. Source: over 2 years ago

Pixel Dungeon and it's many of it's modded versions are free of grinding and financial transactions. Some modded versions have grindy typical RPG aspects added to them, but most don't. Most if not all mods should be available if you are open to using an android emulator. Pixel Dungeon classic is unforgiving and will do it's best to kill you. Shattered Pixel Dungeon is a much more expansive mod of the original that... Source: almost 3 years ago