Mattt Thompson described NSURLProtocol as "an Apple-sanctioned man-in-the-middle attack." Buried deep in Foundation's URL Loading System, NSURLProtocol allows us to intercept requests for URLs from the API libraries we are more familiar with, e.g. AFNetworking, NSURLConnection, UIWebView, NSURLDownload.
Once intercepted, we can "borrow" information from the request/response, add/remove HTTP headers & authentication details, modify the data being transmitted, mock network responses for unit testing and even create entirely new URL schemes. NSURLProtocol has become one of my most powerful tools for working with network connections.