Surge for Mac - Advanced Web Debugging Proxy for Mac & iOS
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
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.
Postman - The Collaboration Platform for API Development
HTTP Headers - HTTP Headers allows you to quickly see the HTTP header information for the current URL.