I remember playing 0ad before. I find it very impressive for a FOSS game.

I am a big fan of roguelikes, mostly Angband. Otherwise I sadly figured out how Steam and Lutris work and started paying for closed-source games...hope, this thread will give me some inspirations :)

Endless sky is cool, I hadn't thought to replace the artwork. My son and I like to play Minetest together, I'll always have a soft spot in my heart for Warzone 2100 Oh and The Dark Mod is fantastic

Here are a few old, open source games for fans of the Bullet Hell genre (all from the same dev). It has been a while since I have played these, but I was addicted to them for a while.

rRootage: This is a vertical shooter where you just fight a series of bosses and dodge their barrage of bullets. It gets very difficult in the later levels. https://www.asahi-net.or.jp/...

TUMIKI Fighters: Like Katamari meets a sidescrolling shooter. This is a weird, but creative twist on a shooter. When you shoot down enemies you can catch them and they will stick to your ship, which lets you use their weapon and the dead ship provides protection (until shot off). Your ship can eventually turn into a giant bullet spewing blob. https://www.asahi-net.or.jp/...

Gunroar: You are a fast moving boat, shooting other boats. https://www.asahi-net.or.jp/...

I personally can't stop playing 0 a.d. It's a great game and the mods are really cool.

One of my all time favorites is Cataclysm: Dark Days Ahead. It's a turn-based survival game set in a post-apocalyptic world. Very active development too, several experimental builds are released every day.