Flarum - Flarum is the next-generation forum software that makes online discussion fun. It's simple, fast, and free.
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.
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
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
NodeBB - NodeBB Forum Software - A better community platform for the modern web. NodeBB is a next generation forum software that's free and easy to use.
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.