Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
RackTables - Racktables is a nifty and robust solution for datacenter and server room asset management.
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.
DCImanager - DCImanager is a platform for managing physical equipment. Connect any physical equipment to a single platform. Use the platform to manage your servers, switches, PDU as well as physical and virtual networks.
Postman - The Collaboration Platform for API Development
Opendcim - a free, web based Data Center Infrastructure Management application.