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Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
httpbin(1) - HTTP request and response service
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.
Endpoints - View and respond to requests on an HTTP endpoint
RegViz - Debugging regular expressions visually
LightStep Sandbox - How quickly can you debug an error in a distributed system?