Craft
Minecraft clone written in C using modern OpenGL
...Many people have used this as a base upon which a more complex game was built. 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!