httptools.dev - Online tools for inspecting and debugging HTTP requests.
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.
Flagsmith - Flagsmith lets you manage feature flags and remote config across web, mobile and server side applications. Deliver true Continuous Integration. Get builds out faster. Control who has access to new features. We're Open Source.
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
RequestHeaders.com - HTTP Header Checker Tools
Fiddler - Fiddler is a debugging program for websites.