I really like WoltLab, they have a great Forum and the community is generally really nice The customer support is also awesome
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.
Kajabi - Kajabi is the only Knowledge Commerce platform today with everything you need to market, sell, and deliver your knowledge online.
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
Tendenci - Tendenci - The Open Source AMS is a cloud-based all-in-one management SaaS solution for Non-profits and Associations.
XenForo - Intuitive. Social. Engaging. Fast. XenForo brings a fresh outlook to forum software.
ChamberMaster - ChamberMaster is a member management software.