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.
Flarum - Flarum is the next-generation forum software that makes online discussion fun. It's simple, fast, and free.
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
Simple Machines Forum - Simple Machines offers free open source software such as SMF, the powerful and easy to use community forum written in PHP. Start interesting discussions on your website!
Zendesk - Zendesk is a beautiful, lightweight help-desk solution.
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.