The "Framework" repository stores general-purpose libraries that are designed for reuse in many different projects. All libraries in here are written to the highest standards of quality: intuitive, modular and efficient.
The .NET assemblies make up the managed part of the framework. These are mature and well-tested (most have >99% test coverage and have seen extensive production use), but these won't receive any big updates anymore as I've turned away from .NET due to its language bloat and sloppy design.
A subset of the .NET assemblies (now found in the Attic folder) were designed for Microsoft's XNA. There shouldn't be any problems using them on MonoGame? as well, but after Microsoft more or less dropped XNA, I have not done any work to test them for compatibility or provide MonoGame? / cross-platform build scripts.
These libraries, having the .Native postfix, are written in cutting edge C++11 and try to do stuff right that existing C++ game engines got wrong.
This is a planned generic game creation tool.