I've been discovering that despite what I originally thought - there exists a small collection of high quality games which are not only free but also developed openly.
I'll list the ones I have found to date, I wouldn't mind seeing any recommendations!
Mindustry - Essentially a fully complete indie game. You can buy it on steam to gain access to a few other features but nothing game changing.
Super Tux Kart - Literally Mario Kart.
Super Tux - Literally Mario
Luanti formerly Minetest. - A voxel game engine, you can add hundreds of mods to make the game into what you want, including into just straight up Minecraft.
Veloren - An open MMO, still very much in alpha development
Thrive - Spore on steroids, still in development although I've spent a good 20 hours having a lot of fun with where it's at, at the moment - with the bacteria stage mostly complete and the multicellular stage taking shape.
Unciv - just civilization.
One thing you probably notice with this list is there are a lot of games which are clones or essentially clones of other games. So I wonder if anyone knows of games which have their own unique style and ideas?
I'll probably post some reviews of each of these games and any more I find out about in separate posts.
Here's a Wikipedia list, but if anyone has any favorites rather than me just dumping a list of varying quality games on to this post.
Check out Freeciv by the way, a libre civilization clone. It's my personal favourite. As for games that aren't clones, there's an Android game called Doors of Doom. I've been thinking about making a game inspired from it for Linux, both mobile and desktop, but I'm not sure how I feel about it yet.