FlatRedBall is described as 'Free and open-source, cross-platform 2D game engine written entirely in C# and based on MonoGame/XNA with additional features on top such as: content management (scenes, objects, variables, text, cameras) 2D physics pathfinding networking camera management scene'. There are more than 10 alternatives to FlatRedBall for a variety of platforms, including Windows, Linux, Mac, iPad and Android. The best alternative is Cocos2d-x, which is both free and Open Source. Other great apps like FlatRedBall are Duality (Free, Open Source), Phaser (Free, Open Source), MonoGame (Free, Open Source) and LibGDX (Free, Open Source).
One framework for creating powerful cross-platform games. The spiritual successor to XNA with 1000's of titles shipped across desktop, mobile, and console platforms. MonoGame is a fully managed .NET open source game framework without any black boxes.
AGKSharp is a free implementation of AppGameKit to be used with C# and .NET With AGKSharp you can write 2D and 3D application and games for Windows in an object-oriented language using the full power of AppGameKit, C# and .
UrhoSharp is a lightweight Game Engine suitable for using with C# and F# to create games and 3D applications. The game engine is available as a portable class library, allowing your game code to be written once and shared across all platforms.
NasNas is a lightweight modular 2D game framework with a focus on pixel art style games. It is entirely written in modern C++17, and doesn't have any dependency other than SFML2. It provides a clean and easy to use API.