tools for developing RESTful web services

What tools are recommended to aid in developing RESTful web services? For instance, a good front end for manually calling REST APIs, and a TCP/IP or HTTP monitoring tool seem like good starting points. I am looking for specific product recommendations. OS X recommendations would also be helpful.

What is REST API tools?

Rest Assured is an API tool that facilitates easy testing of REST services. It's an open-source tool and a Java domain-specific language designed to make REST testing simpler. Moreover, the latest version has fixed OSGi support-related issues. It also offers added support when it comes to using Apache Johnzon.

I use Poster (a FireFox plugin) for manually calling REST web services:

https://addons.mozilla.org/en-US/firefox/addon/2691

(I find the UI is difficult to work with and would like something better.)