MonoGame as it stands today is an open-source implementation of Microsoft's XNA 4.x framework. The platform support for MonoGame ranges from Android and Linux to Windows and even Windows Phone 8 and the OUYA console. Ethan Lee has been maintaining MonoGame-SDL2 as a MonoGame desktop platform built around SDL2. Among the games worked on by Ethan and others that use MonoGame-SDL2 are FEZ, Reus, Rogue Legacy, Blueberry Garden, and Capsized. There's already a large code difference between MonoGame and MonoGame-SDL2, but it may become even larger with Ethan looking at parting ways with MonoGame.
Ethan Lee is looking at a "full fork" of the MonoGame library into a brand new library that "focuses solely on accurate XNA behavior, exclusively for open desktop platforms." Besides focusing upon SDL2 support, the new library would aim to be a lean XNA re-implementation for the desktop and would be very accurate against the XNA 4 framework. Microsoft XNA is the company's run-time environment for video game development on Windows, Windows Phone, and Xbox. XNA is based upon the .NET Framework and has been around for a decade but XNA Game Studio 4 is the last version of the XNA platform that's now being discontinued this year by Microsoft. Ethan also shared he has been rewriting
Those wanting more details on this likely fork to MonoGame can find Ethan's plans via this Google+ post.