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.
vBulletin - Build a community website with the leading forum software, vBulletin 5 Connect, or in the cloud on vB Cloud!
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
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.