Flarum - Flarum is the next-generation forum software that makes online discussion fun. It's simple, fast, and free.
UseResponse - Open-source, self-hosted customer feedback software, live chat and helpdesk system that you can install on your server. Organize documentation using knowledge base and get feedback from social networks with centralized system
ElkArte - ElkArte is a modern, powerful community building forum software.
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.