Entitas is described as 'super fast Entity Component System Framework (ECS) specifically made for C# and Unity. Internal caching and blazing fast component access makes it second to none' and is an app in the Development category. There are four alternatives to Entitas for Self-Hosted solutions, Windows and Linux. The best alternative is EnTT, which is free. Other great apps like Entitas are EntityX (Free, Open Source), Flecs (Free, Open Source) and ECSY (Free, Open Source).
Gaming meets modern C++ - a fast and reliable entity-component system (ECS) and much more. EnTT is a header-only, tiny and easy to use library for game programming and much more written in modern C++, mainly known for its innovative entity-component-system (ECS) model.