Fiddler - Fiddler is a debugging program for websites.
Pingdom - With website monitoring from Pingdom you will be the first to know when your website is down. No installation required. 30-day free trial.
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
StatusCake - Website Uptime Monitoring & Alerts – Free Unlimited Downtime Monitoring
HTTP Toolkit - Beautiful, cross-platform & open-source tools to debug, test & build with HTTP(S). One-click setup for browsers, servers, Android, CLI tools, scripts and more.
Better Uptime - We call you when your website goes down