ManageEngine OpManager - Monitors routers, switches, firewalls, load-balancers, wireless LAN controllers, servers, VMs, printers, storage devices, and everything that has an IP and is connected to the network.
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Cisco Data Center Network Manager - Cisco Data Center Network Manager offers network management system (NMS) support for traditional or multiple-tenant LAN and SAN fabrics.
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...