Hurl.it - Hurl makes HTTP requests.
Postman - The Collaboration Platform for API Development
Graphical HTTP Client - Graphical Http Client is a utility to help you test and interact with REST-based web services.
Sauce Labs - Test mobile or web apps instantly across 700+ browser/OS/device platform combinations - without infrastructure setup.
Cocoa Rest Client - A free open source, native Apple OS X app for testing HTTP/REST endpoints
TestComplete - TestComplete Desktop, Web, and Mobile helps you create repeatable and accurate automated tests across multiple devices, platforms, and environments easily and quickly.