This game offers the same creativity as Minecraft. That means you can break blocks, place them, forge tools and other objects, construct buildings, and build anything that you can imagine. You can do all of this in two game modes: sandbox and normal. In the first one you will have an unlimited cache of blocks and in normal mode you'll have to work hard to survive.
The best thing about Minetest, by all means, is that it lets you play online with friends by creating and building on your own servers. This will let you share space with more than a dozen players; allowing all of you to colaborate in creating any type of building.
Minetest is the best free alternative to Minecraft. The benefits and possibilities don't end there, but continue with your own vision. The best thing you can do is to check out the images and see the limitless potential inside of Minetest.