Veloren is a voxel based MMO game where you can explore the automatically generated environment, fight enemies and group up with other players. It is currently still in development, but it's possible to play at the current point.
The game itself is completely free, and you can download it from the official Veloren website located at https://veloren.net/. Just hit the Download button there, and it will take you to the download page. Choose if you want the most recent stable edition, or prefer the nightly build. The latter option is sort of like a beta test with a lot of bugs and things like that, but will also have more features that are being polished before being released to the stable release of the game.