Zabbix - Track, record, alert and visualize performance and availability of IT resources
Surge for Mac - Advanced Web Debugging Proxy for Mac & iOS
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Weer - A HTTP protocol debugger with Chrome DevTools frontend interface
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.
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy