AzerothCore
Complete Open Source and Modular solution for MMO
...It is written primarily in C++ and provides a complete backend infrastructure for running private game servers, including world simulation, player interaction, combat systems, and database-driven content. The project is highly modular, allowing developers to extend or modify game behavior through plugins and custom modules without altering the core engine. It emphasizes stability and “blizzlike” accuracy, meaning it strives to replicate the original mechanics of the official game as closely as possible. The framework is also used as an educational resource for understanding how large-scale online game servers are structured and maintained. It includes tools for managing server configuration, content scripting, and database operations.