Surge for Mac - Advanced Web Debugging Proxy for Mac & iOS
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
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.
HTTP Headers - HTTP Headers allows you to quickly see the HTTP header information for the current URL.
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.