apimatic 

Common classes used in SDKs generated by APIMATIC as C# .NET Portable Class Library

Unirest is a set of lightweight HTTP libraries available in PHP, Ruby, Python, Java, Objective-C. This is a port of the Java library to .NET, and is done independently and without affiliation to the original Unirest project.

HMACSHA1 and HMACSHA256 implementation for PCL .NET 4.0, Silverlight5, Windows Phone 8.0, Windows Store Apps.

OAuth2Xamarin is a library for adding OAuth2 Three Legged and Two Legged functionality to your application.