Watch out! This tutorial is over 8 years old. Please keep this in mind as some code snippets provided may no longer work or need modification to work on current systems.
Written in C++ (rather than Java like Minecraft itself) Cuberite is a Free and Open Source (FOSS) Minecraft-compatible Game Server. It is designed with performance, configurability, and extensibility in mind, and also aims to accurately recreate most vanilla features.