Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
Fiddler - Fiddler is a debugging program for websites.
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
TrueMetrics Analytics Inspector - Analytics Inspector provides an easy way to validate web analytics and marketing integration, as well as offers a great way to quickly validate the instrumentation via the unique and simplified user interface.
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.