Total Network Inventory: Professional PC audit and network inventory management software - fits best for a for companies of all sizes. Free for 60 days.
Network Inventory Advisor - Network Inventory Advisor audits all network software and hardware simply and efficiently
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.
Lansweeper - Lansweeper discovers Windows, Linux and Mac machines in your network.
Snipe-IT - Managing assets with a Google doc or a shared Excel spreadsheet is more common than you think.
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.
Zabbix - Track, record, alert and visualize performance and availability of IT resources