- Supertux2, it got recently revamped, the quality skyrocketed. Much better controls and artwork.
- Supetux Advance, this is really great too.
- Retux (More Wariolike than Mario)
- Nethack/Slashem. A Roguelike more bound to interaction/exploration/mechanics than combat, but Slashem makes combat crazy with the Doppleganger Monk, which is basically a Shonen Manga, the role. (Dragon Ball/Naruto depending on your age).
- DCSS. Basically, not Nethack/Slashem, much more combat oriented
than the Slashem combinatorics playing with the Monk a la Jackie Chan, this is more like an ARPG made a Rogue.
- Frotz/Lectrote/Winfrotz/whatver Z Machine interpreter and "All Things Devour". Spiritwrak, too. Great libre text adventures and still enganing because of weird mechanics.
- Frozen Bubble
- OpenArena.
- FreeDoom, better compiled with Deutex on daily builds.
- FreeCiv.
- OpenTTD today can be standalone enough.
- Frozen Bubble
- Minetest+tons of subgames such as Glitch, Nodecore...
- OOlite
- Speed Dreams. If the controls are hard, try the arcade mode. If the controls are still hard, get SupertuxKart, pick some real life car from the addons and get all the SD tracks from the inline downloader, they are several.
Heads up: they recently changed name to Luanti to get away from the "it's Minecraft but worse" perception. They also un-bundled the built-in game and are trying to be a game engine these days.
I recommend looking into Age of Mending. It's still in alpha, but if it's ever finished it'll give Minecraft a good run for its money, especially among the builder-minded players.
Luanti had an annoying habit of regenerating terrain while I was still walking on it. In my book it is very much still "minecraft but worse." I was on a very weak ARM device though.
Regenerating? Luanti doesn't ever regenerate terrain. Do you mean generating the first time? Or loading? I've had it happen that I run into the edge of the mapblock and can't walk into the next one because it's loading or still generating. Maybe give it another chance, because I can't picture the behavior you're describing.
OpenArena even has a browser version these days but sadly it doesn't seem to have any active servers anymore. I had progressed to the point where I could strafe jump and rocket jump all day.
The amount of care they put into the little quality of life ui stuff is impressive. Something about how you can tell when the developers enjoy playing their game.
Also Brad Smith is a heavyweight in the NES ROM/Emulation scene, once you know his name you’ll start to notice it pop up all over the place. (e.g. nsfplay)
The Free Civ and Free Colonization games are good. Brogue, Nethack, DCSS are good if you like roguelikes. OpenMW is a totally open source reimplementation of Morrowind, so that might fit the bill.
Oh, yeah! And I believe there are not many games which have Linux, Windows and MacOS versions allowing interplay. Several years ago we had one or two LAN parties with hardware running all three operating systems.
https://www.beyondallreason.info/ is high quality and in active, open development. And fun. For some weird reason 8v8 is the default experience but there are several other modes: PVE, 1v1, etc.
- Starcraft Brood War - though I have not tested it via the battlenet.exe, played it when it still was payware.
- Gemcraft Labyrinth - played it when flash was a thing, but now it is again, e.g. on Armorgames
And, I got games like Portal 1 & 2, Outer Wilds, Osmos, Manifold Garden, Planet Crafter and many more for under ten, some for under five Euros each when they were on special offer on Steam. I once got Just Cause 3 for three Euros.
Stardew Valley isn't open source, but it has been decompiled (.Net) and has a an open-source modding API inclding Harmony integration for patching the base game code and and a huge open-source modding community.