Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
RackTables - Racktables is a nifty and robust solution for datacenter and server room asset management.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
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.
Fing - Discover which devices are connected to any Wi-Fi network, map devices, detect intruders, assess...
NetBox - NetBox is an open source web application designed to help manage and document computer networks. NetBox was developed specifically to address the needs of network and infrastructure engineers.