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.
Taiga - Project management platform for agile developers & designers
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
TargetProcess - Agile Project Management Web Application
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Craft.io - All-in-one product management software for entire product lifecycle