osTicket - osTicket is a widely-used open source support ticket system.
UserVoice - UserVoice integrates easy-to-use feedback, helpdesk, and knowledge base management tools in one platform that empowers users to speak and companies to understand.
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
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Canny - Canny helps you collect and organize feature requests to better understand customer needs and prioritize your roadmap.