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.