Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
OpsDash - All-in-one solution for monitoring servers, services, uptime and databases. Curated Dashboards. Rule-based alerting.
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
Monitornator - Server monitoring made simple
Postman - The Collaboration Platform for API Development
NIXStats - SaaS monitoring tool for window/linux servers and websites. Think up-time robot but with more features.