I really like WoltLab, they have a great Forum and the community is generally really nice The customer support is also awesome
Discourse - Discourse is an open source discussion platform built for the next decade of the Internet.
LimeSurvey - LimeSurvey: the Online-Umfrage Tool - Open-Source Suryeys
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.
Typeform - Create beautiful, next-generation online forms with Typeform, the form & survey builder that makes asking questions easy & human on any device. Try it FREE!
Flarum - Flarum is the next-generation forum software that makes online discussion fun. It's simple, fast, and free.
Google Forms - Simple web forms from Google.