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.
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
AppDynamics - Get real-time insight from your apps using Application Performance Management—how they’re being used, how they’re performing, where they need help.
HTTP Headers - HTTP Headers allows you to quickly see the HTTP header information for the current URL.
Zabbix - Track, record, alert and visualize performance and availability of IT resources