Fiddler - Fiddler is a debugging program for websites.
LiveReload - LiveReload 2 proudly presents… The Web Developer Wonderland. (a happy land where browsers don't need a Refresh button). CSS edits and image changes apply live. CoffeeScript, SASS, LESS and others just work.
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
Browsersync - Browsersync makes your tweaking and testing faster by synchronising file changes and interactions...
HTTP Toolkit - Beautiful, cross-platform & open-source tools to debug, test & build with HTTP(S). One-click setup for browsers, servers, Android, CLI tools, scripts and more.
Ghostlab - Ghostlab allows you to test out a newly developed website on a variety of browsers and mobile devices at the same time. To get started, simply drag the web address to the Ghostlab system and press the play button. Read more about Ghostlab.