MineCraft will be open source someday in the future (http://www.minecraft.net/about.jsp):
"Once sales start dying and a minimum time has passed, I will release the game source code as some kind of open source. I'm not very happy with the draconian nature of (L)GPL, nor do I believe the other licenses have much merit other than to boost the egos of the original authors, so I might just possibly release it all as public domain. "
But for now, enjoy this list of open source MineCraft alternatives!
"Manic Digger is a public domain 3d block-building game similar to Minecraft."
Features: Infinite world, Rails, Crafting, Shadows, Multiplayer
License: Public Domain
2. Opencraft: http://opencraft.sourceforge.net
OpenCraft is an alternative Minecraft server, providing more advanced features than the official distribution.
Features: Scripting, griefing preventation
3. Minedroid: http://code.google.com/p/minedroid/
Features: Work with a touchscreen interface
License: New BSD
4. Infiniminer: http://thesiteformerlyknownas.zachtronicsindustries.com/?p=713
Features: Survival mode, creative mode, multiplayer, dynamic lighting, almost infinite world, Infinite map generator
6. TechCraft: http://techcraft.codeplex.com/
Features: Minecraft style terrain engine
7. ArdorCraft: http://ardorlabs.se/ArdorCraft/
Features: API to build new types of blockworld games
8. Terasology (Blockmania):
Features: Procedural terrain generation, efficient rendering
License: Apache 2.0
Finally, this one is not open source, but you can't miss it!
http://ace-spades.com/ (based on voxlap engine)
"It's the best bits of Minecraft crossed with the fast-paced shooter gameplay of Team Fortress."
Features: Dynamic lighting, 32 players battling, CTF or Territory Control game modes
License: Closed Source
Ace of Spades is now a commercial game, but we have
9. Iceball: Open Source Version of Ace of Spades!https://github.com/iamgreaser/buldthensnip
10. Minecraft in Blender: Minecraft like in the Blender Game Engine.
Blackvoxel (C/C++, GNU GPL V3)
AIR-based Minecraft Map Viewer (with source code):
Minecraft like games in flash:
A minecraft level render in flash:
Source Code of MCExplorer (open source minecraft map render/viewer in flash/as3):
Minecraft and WebGL:
Minicraft(update on 2013/11/15, html5 in 252 bytes with source code):
http://gameslikefinder.com/games-like-minecraft/(Thanks to Samuel Taylor)