osTicket - osTicket is a widely-used open source support ticket system.
Office Timesheets - Professional Services Automation
Zammad - The Zammad - Help Desk and Ticket System - Community Page
HarmonyPSA - HarmonyPSA - Professional Services Automation (PSA) for IT Businesses
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Timewatch - Timewatch offers time tracking, expense management, resource scheduling and professional services automation solutions.