Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
PreFlight - Preflight is an Automated Web Testing tool. Allows everyone to automate Web UI tests.
httpbin(1) - HTTP request and response service
HTTP Status Dogs - Hypertext Transfer Protocol Response status codes, and dogs
Endpoints - View and respond to requests on an HTTP endpoint
HTTP Status Cats - Cats for every HTTP status code, by ex-Google.