osTicket - osTicket is a widely-used open source support ticket system.
Zammad - The Zammad - Help Desk and Ticket System - Community Page
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
JitBit Helpdesk - JitBit is a web-based help desk and ticketing software solution
ZenDesk - Zendesk is a beautiful, lightweight help-desk solution.
UseResponse - Open-source, self-hosted customer feedback software, live chat and helpdesk system that you can install on your server. Organize documentation using knowledge base and get feedback from social networks with centralized system