Decoder - Instantly decode error and status codes
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
HTTP Status Dogs - Hypertext Transfer Protocol Response status codes, and dogs
Endpoints - View and respond to requests on an HTTP endpoint
HTTP Status Tester - Test URLs in bulk for their HTTP responses, by ex-Google
httpbin(1) - HTTP request and response service