MonoGame is a free C# framework based on Microsoft's XNA game development tools. It allows you to use Microsoft's .NET programming languages to make games for any platform or operating system. It has a great, active community, and allows you lots of flexibility in creating your game.
Games like Celeste, Towerfall and Fez were made with MonoGame.
- Check out this MonoGame tutorial series from game from scratch.
- Word of Zero also has a tutorial video on getting started with MonoGame.
- There is also a great blog post about the tool on Microsoft devblogs.
- Get the tool here.