osTicket - osTicket is a widely-used open source support ticket system.
Bitrix24 - Bitrix24 is a free (for small businesses) social enterprise platform. It is a united work space which handles the many aspects of daily operations and tasks. Create your own social intranet in minutes!
Zammad - The Zammad - Help Desk and Ticket System - Community Page
VOGSY - Professional service automation tool built on Google G Suite
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Office Timesheets - Professional Services Automation