Loader.io is a free load testing service that allows you to stress test your web-apps & apis with thousands of concurrent connections.

Webomates provides cloud-based Testing as a service (TaaS) to leading software companies

Browser and Mobile Automation Test Framework for Node.js

Professional software tool for HTTP testing and debugging

Hand-crafted lightweight Rest Client for testing APIs. Supports collections, environments, git collaboration and local storage.

Burp or Burp Suite is a set of tools used for penetration testing of web applications. It is developed by a company named Portswigger who make freely available a Web Security Academy which has a range of lessons on penetration testing.