I see this thread is occasionally updated and bumped with new stuff, so I guess I'll add my own UI lib to this list as well.
GeonBit.UI is a full UI system for MonoGame projects that comes with 2 default themes (both are old-school styles), and can be easily extended to create your own special styles. Here's the git repo:
A quick demonstration video:
And installation tutorial:
If you want to discuss it there's currently an open thread here http://community.monogame.net/t/geonbit-ui-ui-extension-for-monogame/8529