Noesis(link http://www.noesisengine.com/) is a GUI framework based on the same technique as Microsoft's WPF (Windows Presentation Foundation), but with cross-platform abilities and more focused on good rendering performance. It is used for extremely flexible GUI solutions, mostly for games and other realtime 3D-applications, but it is equally well suited for general applications.
In my view this would be the perfect companion to Mono/Xamarin for cross-platform applications.
I have been using WPF for several years, and it is by far the most flexible and well-done GUI framework I have ever used. There are two major shortcomings, however - it is Windows only, and it suffers a bit performance-wise if you make huge, realtime-updating GUIs.
Noesis seem to adress both those very issues. There is a trial version of the plugin they make for the Unity game engine, and I've tried it out. It is definitely the absolutely best GUI plugin for Unity - and there are many to choose from.
Being able to utilize the WPF based techniqe of Noesis for our games made in Unity is fantastic. It would, however, be even more fantastic if we could use Noses with Mono/Xamarin.
Noesis have been in contact with Xamarin discussing if a version of their GUI framework would be of interest for the Mono/Xamarin community. I ask you to check this technique out and spread the word. I for one put my vote in for Noesis+Mono/Xamarin to happen.