AskBot - Askbot enables users to create Q&A forums.
The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.
OSQA - OSQA is the free, open source Q&A system you've been waiting for.
Scoold - An open-source, enterprise-friendly Q&A platform inspired by Stack Overflow.
Stack Exchange - Stack Exchange is a fast-growing network of 84 [and counting] question and answer sites on diverse...
Super User - Super User, part of https://alternativeto.