There is one that was just recently announced and is currently available on GitHub. It's called Portable Rest
https://github.com/advancedrei/PortableRest
PortableRest is a Portable Class Library for implementing REST API
clients in other Portable Class Libraries. It leverages JSON.NET for
rapid, customizable serialization, as well as the Microsoft.Bcl.Async
library for awaitable execution on any platform. It is designed to be
largely drop-in compatible with RestSharp, though you will need to
make some changes and recompile.
This initial release has limited support for simple JSON requests.
More options (including XML and hopefully DataContract support) will
be available in later releases.
