It's clear that Minecraft is a favourite game for millions. Because it is written in Java, it can run on a variety platforms, including Linux. There are many Minecraft tinkerers who would love to have access to the source code and get to work on it. Unfortunately, the source code is not accessible to the general public.
There's good news. Many people have tried to recreate Minecraft and other similar games using open-source software. Are you looking for a free Minecraft alternative to Minecraft? Here's a quick overview of some Minecraft clones and derivatives that you should really check out.
These projects are at a variety of stages of completion and serve many different purposes. Some attempt to replicate the Minecraft experience entirely, while others aim to provide a very similar experience. Others are pushing the voxel-based gaming idea in completely new directions. Still others are more of a framework that can be used to help you create your own game or creation.
Minetest is the first game on our list. Minetest, a "near infinite-world block sandbox sandbox game" and a "game engine", is the most complete alternative to Minecraft. It supports both multiplayer and subgames. It also features a variety of terrain generators as well as different default biomes. It also has a very user-friendly API to create mods in Lua.
Minetest is open-source under the LGPL. It is written primarily using C++, which makes it much faster than other scripting languages. Minetest can be used on Windows, OS X and Linux as well as Android, Linux, Android and FreeBSD. You can find the source code at GitHub.
Minetest screenshot, Minetest website, CC-BY-SA 3.0.
Terasology may be the winner of the best rendering engine award in the pack. Its shadows are both beautiful and ominous. This game started as an experiment in procedural terrain creation. It has evolved into a fully-featured game with multiplayer and a variety of add-on modules that allow you to try different gameplay mechanics.
Terasology is written using Java and made freely available under the Apache 2.0 license. It is Java-based and should run on any platform that has sufficient power.
Terasology screenshot, Terasology Code repository, Apache 2.0.
Voxel.js can be used in a variety of related projects. This allows you to use as much or little code as you like when creating your game. Although the main library, voxel engine, is a basic engine that renders boxy scenes well, there are more than 200 add-ons. You can see examples of other engines in the gallery. The main engine is released under the BSD-style license. Other add-ons might be licensed differently so make sure to check before you make assumptions.
Screenshot of voxel forest using Voxel.js by Jason Baker
TrueCraft is designed to be very similar to the original game. It is described as a Minecraft implementation, rather than a clone. It is compatible with official Minecraft server releases. TrueCraft's author seeks to implement beta version 1.7.3, which he considers "nearly perfect" during Minecraft's development. TrueCraft is a snapshot that has been intentionally frozen in time. He seeks feature-parity to Minecraft.
TrueCraft is very close to the original. Therefore, TrueCraft has taken great care to avoid copyright issues. TrueCraft allows code only from developers who haven't decompiled or otherwise had access the original game source code. However, those who have are welcome and encouraged to contribute in other ways. TrueCraft is written using C# and is open-source under an MIT License.
Craft is an open-source voxel engine that mimics Minecraft. Although development seems to have slowed down or stopped, there are more than 200 forks, many of which (such as the school project Not2bad-craft), offer major improvements. If you are interested in creating a Minecraft-like game, Craft's simplicity might appeal to you. The game engine is just a few thousand lines in C code and uses OpenGL to render. It uses simple algorithms to generate terrain and other tasks. Everything is stored in an SQLite3 databank. It's worth looking into a Python-based multiplayer server.
Craft is available under an MIT licence
You should also check out these other notable mentions:
Freeminer, another sandbox-based game, is inspired by Minecraft and based upon Minetest. The authors aim to make the game enjoyable while still maintaining some perfectionionism. It comes with installers for Android, Windows, and Linux. ClassiCube is a Minecraft Classic clone in C#. It is open-source under the OpenTK license and can be installed on Linux, Solaris, Windows OS, Solaris, Windows OS, Mac OS and in a browser.
There you have it. This is not a complete list. There are many more options available that you can explore. As more people start to play these games, or create their own, we will undoubtedly see more options. Which one do you prefer? And which one did we miss that you wish we had?