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