Cataclysm DDA is amazing if you are into this stuff.
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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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).
Crack Attack! is a loving parody of addictive tetrislike games that is also an extremely addictive tetrislike game.
I liked oolite (space trading and combat game, also DRL (aka doomRL) and cataclysm DDA , also there is The Ur-Quan Masters megamod (A 2d space game)
Big fan of Quaver, its a community-driven, and open-source competitive rhythm game.