ezEngine

ezEngine is a free, open source game engine written in C++. Its philosophy is to be modular and flexible, such that it can be adapted to many different use cases. EZ utilizes a plugin system to integrate features such as FMOD or Jolt Physics. This makes it possible to only include those features that you need, or to replace systems with a custom solution that works better for your requirements.

Similarly, the EZ code base can be built in multiple tiers, where you either get the entire feature set, with a fully functional editor, asset management and renderer, or you can strip it down to just the base libraries and core engine functionality. This can be extremely useful if you need to build a lot of custom technology, but require a high-performance, reliable foundation. EZ has a strong emphasis on providing robust, easy to use and well-tested base functionality. It is successfully being used in such a capacity in commercial products.