StopLight!

Fiddler is a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP(S) traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler includes a powerful event-based scripting subsystem, and can be extended using any .NET language.

Postman helps you be more efficient while working with APIs. Postman is a scratch-your-own-itch project. The need for it arose while one of the developers was creating an API for his project. After looking around for a number of tools, nothing felt just right. The primary features added initially were a history of sent requests and collections. A number of other features have been added since then. Here is a small list.

Eviware, the maker of soapUI, Web Service Testing Leader

The web developers helper program to create and test custom HTTP requests.Advanced REST client for Google Chrome.

A REST client for almost any web service (Firefox and Chrome extension).

Runscope helps you monitor, log and measure API usage to solve API problems fast. Automatically monitor APIs without any coding. Runscope provides complete logging for all of your API traffic. Track down troublesome API calls with powerful search. Debug problems by peering into the details of every request and response. Works with any public or private API, behind the firewall or in the cloud.

apptalk.ninja is a great, easy-to-use tool for app developers tackle that monitors and debugs issues with their apps’ communication over-the-air.

APICombo is a user-friendly API and website testing and monitoring tool.

Automated API testing for the entire lifecycle. No coding, web-based, actually works.

RESTtest is a simple yet intuitive way to test RESTful web services and standard HTTP calls. RESTtest allows you to enter a HTTP request body and any necessary headers. After the request is submitted, response information can be both viewed and saved to disk.