Watch out! This tutorial is over 10 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.