Recs.
Updated
MonoGame is a C# framework based on XNA, it can run on Windows, Mac, Linux, Mobile and Console targets. It is used by the developer of Stardew Valley. A slimmed down version is used for the Pixel Vision 8 retro game engine.
Read What is the best game engine: is MonoGame right for you?.
SpecsUpdate
Pros
Pro Cross-platform
Support for iOS, Android, Mac OS X, Linux, Windows (both OpenGL and DirectX), Windows 8 Store, Windows Phone 8, PlayStation Mobile, PlayStation 4, Xbox One, Nintendo Switch, and the OUYA console with even more platforms on the way.
Cons
Con Slow rate of updates
Versions 3.9 is overdue by a year, and version 4.0 is set to release in 2040.
Con Non-Windows tools are a bit funky
Monogame support for Xamarin Studio or Monodevelop is a bit shaky especially for library references. Only good non-Windows IDE compatible with MonoGame is Rider and that costs money & isn't open-source.