Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
RegExr - RegExr.com is an online tool to learn, build, and test Regular Expressions.
Endpoints - View and respond to requests on an HTTP endpoint
Regulex - Open-source, regular expression parser and visualizer.
httpbin(1) - HTTP request and response service
Nudge for Prototypes - Catch usability issues and design mistakes pre-deployment.