osTicket - osTicket is a widely-used open source support ticket system.
Freshdesk - Freshdesk is a cloud-based customer support software that lets you support customers through traditional channels like phone and email, social channels like Facebook and Twitter, and your own branded community
Zammad - The Zammad - Help Desk and Ticket System - Community Page
LiveAgent - LiveAgent is a fully-featured web-based live chat and help desk software. It harnesses the power of a universal inbox, real-time live chat, built-in call center, and a robust customer service portal. Start your free 1 month trial today!
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Zoho Desk - Industry's first context-aware Helpdesk Software