Federacy - Bug bounty and security testing platform.
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
IssueHunt - IssueHunt is a crowd funding service for opensource contributors and owners!
httpbin(1) - HTTP request and response service
HackerOne - HackerOne provides a platform designed to streamline vulnerability coordination and bug bounty program by enlisting hackers.
Endpoints - View and respond to requests on an HTTP endpoint