Postman - The Collaboration Platform for API Development
Zabbix - Track, record, alert and visualize performance and availability of IT resources
StopLight - Stoplight is an API Design, Development, and Documentation platform that enables consistency, reusability, and quality in your API lifecycle, all with an easy, enjoyable developer experience.
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.
Mockoon - Mock servers in seconds
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.