Postman - The Collaboration Platform for API Development
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.
Ping-API - Automated API testing and monitoring.
Ettercap - Ettercap is a suite for man in the middle attacks on LAN.
HTTPie for Terminal - Developers all around the world rely on HTTPie for API testing.
tcpdump - tcpdump is a common packet analyzer that runs under the command line.