CodeHub is an iOS application written using Xamarin

One framework for creating powerful cross-platform games.

Entitas is a super fast Entity Component System (ECS) Framework specifically made for C# and Unity

Desktop runtime for apps built on web technologies, using the system's own web browser engine

Cross platform GUI framework for desktop and mobile applications in .NET

FNA - Accuracy-focused XNA4 reimplementation for open platforms

PeachPie - the PHP compiler and runtime for .NET and .NET Core

Carter is framework that is a thin layer of extension methods and functionality over ASP.NET Core allowing code to be more explicit and most importantly more enjoyable.

Extensions to make MonoGame more awesome

Qml.Net - Qt/QML integration/support for .NET

A cross-platform UI toolkit for creating desktop applications with .NET and Mono

UI Library for MonoGame, FNA and Stride

The Ultraviolet Framework is a .NET game development framework written in C#.

UI system for MonoGame projects.

MonoGame.Forms is the easiest way of integrating a MonoGame render window into your Windows Forms project. It should make your life much easier, when you want to create your own editor environment.

Incredibly small finite state machine for .NET languages, written in C#. Useful for UIs and games. Runs in .NET, mono, Xamarin and Unity. Specially useful with Unity3D and MonoGame.

MonoGame Content Importers, Shaders, etc

It's a compound of WindowsForms, MonoGame, FarseerPhysics and Penumbra and it shows the possibility of creating something like an editor inside a windows forms control.

Rebuilding the Legacy GUI from Unity3D for MonoGame