Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
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.
Fing - Discover which devices are connected to any Wi-Fi network, map devices, detect intruders, assess...
Postman - The Collaboration Platform for API Development
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use