Minosoft
An open source Minecraft reimplementation written from scratch
...The goal of Minosoft is to provide a more flexible and stable architecture that developers can experiment with while exploring alternative implementations of the Minecraft client. Because it is written with modern programming tools and languages, the project aims to improve reliability and extendability compared with traditional client software. Minosoft also serves as a development platform where programmers can experiment with networking, rendering, and gameplay systems used in voxel-based sandbox games.