[frik]

A similar Minecraft "clone" in C++. The author writes blog articles about the development, very interesting read!

http://sea-of-memes.com/LetsCode4/LetsCode4.html

[mistermumble]

Minetest is a C++ based Minecraft-style simulation with a clean API for modding -- with a nice little ecosystem of mods.

(I am not affiliated with them. I just downloaded it a few months ago and have been tinkering with it.)

http://minetest.net/