Games
0AD
Cataclysm: Dark Days Ahead
Personal comment:
I find it hard to start with but it is becoming fun over time. Surprisingly, there's a lot of variety here.
Dwarf Fortress
Personal comment:
If you want to get started, you're free to lurk in this subreddit and look for their getting started guide.
id Software's Games
Megaman Maker
Minetest
Personal comment:
The vanilla version is quite bare and it doesn't feature the feature set Minecraft has (like mobs, for example). You could expand it through add-ons.
OpenTTD
Super Tux Kart
Personal comment:
One of the best free and open software game I've ever played. Fully recommend.
Ultima Ratio Regum
Powder Toy
Veloren
Personal comment:
As of 2019-07-17, it is under You can get in touch with the community through their subreddit and their Discord server. Also as of 2019-07-17, the game is under early development so its features is not yet complete compared to Cube World.
Personal comment:
This is one of the grandest free and open source game out there. From its presentation to its gameplay. And as of writing this (on 2019-07-24), it is still under early development so expect instability when first starting this game.