A very performant Minecraft clone written in just a few thousand lines of C using modern OpenGL (shaders). Online multiplayer support is included using a Python-based server. Features:

Simple but nice looking terrain generation using simplex noise. More than 20 types of blocks and more can be added easily. Supports plants (grass, flowers, trees, etc.) and transparency (glass). Simple clouds in the sky (they don't move). Day / night cycles and a textured sky dome. Signs - you can write text on any block. Lights - any block can be set as a light source. Ambient occlusion for basic shading of blocks. World changes persisted in a sqlite3 database. Multiplayer support!