pacman?

• Dwarf Fortress - colony sim
• CDDA - rougelike
• FreeDoom - Doom port, compatible with most mods
• Lugaru - ninja rabbit fighting
• Minetest - minecraft clone
• Super Tux Kart - racing game
• Daggerfall Unity - rpg
• Pathos - nethack clone rougelike
• Endless Sky - space sim
• Hedgewars - worms like game with hedgehogs
• Battle for Wesnoth - strategy
• ColoBot - programming puzzle game/rts
• OpenTTD - transport tycoon
• Shattered Pixel Dungeon - rougelike
• Mindustry - like factorio but more tower defense
• 0.A.D - rts

these are the ones i've tried

oh and keep an eye out for Kitten Space Agency

Battle for Wesnoth

0 AD

Widelands

FTL is great. It'll probably run on a toaster. I've also heard that it has a bajillion great mods to play if you get tired of the base game

rimworld, project zomboid, starsector...

Stardew Valley

Factorio.

Dungeon Crawl Stone Soup

Free: Battle for Wesnoth is really great, I haven't played it in a long time but it was already great like 10-15 years ago so it's probably even better today, Nethack (if you don't mind the starting difficulty and the "graphics") is also great, VERY complex gameplay but very rewarding if you know it fairly well. Also saw a video of Xonotic today, looks also really good if you're into fast multiplayer arena shooters (Quake-like). Heard positive things about 0 AD as well (Age of Empires-like). All of these are open source and in the extra repository on Arch.

Non-free but really cheap: Stardew Valley is probably great, I've never played it and it doesn't look like my cup of tea but I've only heard positive things plus it's like #1 or #2 rated on Steam, so it must be really good.

Non-free: Stellaris (got into it recently, great game and well-maintained Linux client (not at all common), much better than I expected, VERY complex and content-rich, quite expensive when you want all DLCs. It's like a live service game, you'll pay quite a bit if you want everything, but you also get tons of content). Also, Alien Isolation is one of my favorite single player horror games of all time and it also has a Linux client (which was a surprise for me) but that one is probably outdated and not maintained anymore by now I'd guess (but didn't look it up) so it might be better to play the Windows client via Proton. I'm not up to date on that though - look it up. Oh, and POOLS also has a Linux client, that's a great small walking simulator, "Backrooms"-like, very atmospheric and great visual design.

Rhythm Doctor. Sadly it needs XWayland ;(

Doom

You could try searching Archive.Org for Linux native games