HTTP Debugger - Debug HTTP API calls to a back-end and between back-ends. Easy of use, clean UI, and short ramp-up time. Not a proxy, no network issues!
Surge for Mac - Advanced Web Debugging Proxy for Mac & iOS
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
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.