I really like WoltLab, they have a great Forum and the community is generally really nice The customer support is also awesome
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
phpBB - Raspberry Pi. The Raspberry Pi is a cheap, credit-card sized computer. The official website uses phpBB for their discussion forums. phpBB is not affiliated with nor responsible for any of the sites listed on the showcase.
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.
Survicate - Collect feedback on your website and find out more about your visitors.
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.