httpbin(1) - HTTP request and response service
CONV. - Convert YouTube video to MP3, GIF or MP4
HTTPDump - A simple, beautiful Webhook debugging tool.
Downie - Video download for OS X. Downie supports at this moment over 1,000 different sites.
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
Yout - YouTube to Mp3 Converter