Invision Community - Invision Community is a flexible, integrated set of apps for building communities on the web.
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.
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.
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.