RackTables - Racktables is a nifty and robust solution for datacenter and server room asset management.
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.
Opendcim - a free, web based Data Center Infrastructure Management application.
Snipe-IT - Managing assets with a Google doc or a shared Excel spreadsheet is more common than you think.
EasyDCIM - EasyDCIM Is an Control Panel For Automated Data Center Management By ModulesGarden.
Zabbix - Track, record, alert and visualize performance and availability of IT resources