Free and Open Source Software
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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Cataclysm: Dark Days Ahead is pretty astounding, offering an incredible level of detail and degree of freedom, plus just lots of zombies to fight.
Crack Attack! is a loving parody of addictive tetrislike games that is also an extremely addictive tetrislike game.
Definitely Neverball. My kids and I spent so many hours in it.
OpenTTD is good, so is TuxKart, but both have better closed-source alternatives. I don't think Neverball does.
I really like MineClone2 (a Minetest game).
Lix – https://www.lixgame.com/
FOSS Lemmings, with many great features (like multiplayer) and active development.