NodeCore is another great Minetest game. I haven’t been able to find a game that matches the feeling of discovery it provides as you learn about the rules of the world.
I don’t know if it counts, but I think the source code of Freespace 2 was released eventually und a non-profit license. The community did some great things with it, especially the Blue Planet campaign.
Happily, that’s no longer the case these days. TDM was originally a mod, but despite keeping the name, it’s now a standalone game using the open-sourced Doom 3 engine. The whole game is currently free without any purchases necessary!
Armagetron Advanced - a Tron light cycles clone that was a blast for a long time. They even released for free on steam. I’ve not played in a long time, but now want to jump back on
I haven’t thought about this game since I was in junior high. Wow. I should look into it again since guitar hero and rock band have become so inaccessible. Used guitar controllers going for $150+. Fucking outrageous.