Total Network Inventory: Professional PC audit and network inventory management software - fits best for a for companies of all sizes. Free for 60 days.
Zabbix - Track, record, alert and visualize performance and availability of IT resources
Network Inventory Advisor - Network Inventory Advisor audits all network software and hardware simply and efficiently
Datadog - See metrics from all of your apps, tools & services in one place with Datadog's cloud monitoring as a service solution. Try it for free.
Lansweeper - Lansweeper discovers Windows, Linux and Mac machines in your network.
Prometheus - An open-source systems monitoring and alerting toolkit.
Nmap - Nmap Free Security Scanner, Port Scanner, & Network Exploration Tool. Download open source software for Linux, Windows, UNIX, FreeBSD, etc.