The options I'm aware of are:
https://github.com/picoe/Eto (OP - BSD)
https://github.com/AvaloniaUI/Avalonia (MIT)
Currently only with a preview for Mac, Xamarin.Forms is roadmapped for desktop platforms "Q3 2017": https://blog.xamarin.com/glimpse-future-xamarin-forms-3-0/
Licensing for Xamarin Forms was funky for a while (required paying for Xamarin prior to MS acquisition) but appears to be MIT now.