Hoppscotch : A free, online, open source API request builder
Phantomas : PhantomJS-based web performance metrics collector and monitoring tool
The Coach : The coach helps you find performance problems on your web page. Think of the coach as a modern version of YSlow. The coach will give advice of how you...
WebScarab : WebScarab is a framework for analysing applications that communicate using the HTTP and HTTPS protocols.
Browser Calories : The easiest way to measure your performance budget.
The Web Developer extension adds various web developer tools to a browser. The extension is available for Chrome, Firefox and Opera, and will run on any platform that these browsers support including Windows, OS X and Linux.