httptools.dev - Online tools for inspecting and debugging HTTP requests.
RequestHeaders.com - HTTP Header Checker Tools
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.
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!
Live HTTP Headers - The goal of this project is to adds information about the HTTP headers in three ways:
HttpWatch - HttpWatch: The ultimate HTTP Sniffer for Chrome, IE and iOS