osTicket - osTicket is a widely-used open source support ticket system.
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.
Zammad - The Zammad - Help Desk and Ticket System - Community Page
Snipe-IT - Managing assets with a Google doc or a shared Excel spreadsheet is more common than you think.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Zabbix - Track, record, alert and visualize performance and availability of IT resources