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.
The Dude - MikroTik is a Latvian company which was founded in 1996 to develop routers and wireless ISP systems. MikroTik now provides hardware and software for Internet connectivity in most of the countries around the world.
Snipe-IT - Managing assets with a Google doc or a shared Excel spreadsheet is more common than you think.
Pandora FMS - Pandora FMS: Opensource monitoring software
Zabbix - Track, record, alert and visualize performance and availability of IT resources
cStatus - Monitor and scan a network.