gURL - gURL is a free, Open Source (GPL) HTTP request generator tool.
Graphical HTTP Client - Graphical Http Client is a utility to help you test and interact with REST-based web services.
Postman - The Collaboration Platform for API Development
soapUI - SoapUI Pro is one of the most prominent API testing platforms around, allowing developers to quickly prototype the functions of their apps and get them to market with little hassle.
Hurl.it - Hurl makes HTTP requests.
RESTClient - RESTClient can be used to test variety of HTTP communications and RESTful webservices.