MineClone2 is pretty much as close as you can get to Minecraft without playing Minecraft. It's free, open source and runs on the Minetest voxel game engine. A new release is now available with version 0.81.0.
Minetest is one of those things that I always feel like deserves more attention, it's not Minecraft, but it is so much more flexible and with how moddable it is has a lot more potential imo
Rassilonian Legate
in reply to Liam @ GamingOnLinux 🐧🎮 • • •(I say this as an ex Minecraft addict)