Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
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.
Angry IP Scanner - Open-source and cross-platform network scanner designed to be fast and simple to use
RackTables - Racktables is a nifty and robust solution for datacenter and server room asset management.
Advanced IP Scanner - Advanced IP Scanner shows all network devices, gives you access to shared folders, and can even remotely switch computers off. Download it Free.
Opendcim - a free, web based Data Center Infrastructure Management application.