Get Start with Xamarin.Form
Open Source Components for Xamarin, has lots components for iOS and android or windows, such as Barcode Scanner, Bluetooth LE, Device Info, Fingerprint, Geolocator, Media. All these components are very helpful, if you choose to build your own then you have to imprement DependencyService, create interface at PCL project and imprement at each device, which is a lots work.
Few things I think they need to improve are, more detail dodumentation, most of them has document and sample code, but sometime it's also what's dependency should I do if I use this library, such as registered google API, registerd Firebase to create a project.
Xamarin Samples, you can find lots of example from Xamarin.
Awesome-xamarin [Github], this github has lots useful information relative with Xamarin.Form.