I can recommend colobot if you like to automate things with code.
Cataclysm: Dark Days Ahead is pretty astounding, offering an incredible level of detail and degree of freedom, plus just lots of zombies to fight.
Cataclysm DDA is amazing if you are into this stuff.
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).
Other than the Valve made stuff, I think my favorite Source engine game is Dark Messiah of Might and Magic. Kicking orcs off cliffs or into spikes is super fun and the basic movement on the engine is pretty much the best FPS has.
Big fan of Quaver, its a community-driven, and open-source competitive rhythm game.
Lix – https://www.lixgame.com/
FOSS Lemmings, with many great features (like multiplayer) and active development.
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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