HttpClient is a modern networking API that makes it easy to access any resource exposed through HTTP. The HttpClient API has been available in some versions of the .NET Framework for a while now. This NuGet package makes a standard set of HttpClient APIs available across a wider array of platforms, including Windows Phone 7.5 and Windows Phone 8, .NET Framework 4.0 and later versions, and Windows Store apps. This way you can share more .NET code across these platforms.
This package also addresses one of the top requests from the UserVoice forum for .NET: providing an HttpClient implementation for Portable Class Libraries (and therefore also on Windows Phone 8).
For more info, see Get /httpclient/rtm – 200 OK on the .NET Framework’s blog.