OSQA - OSQA is the free, open source Q&A system you've been waiting for.
Stack Overflow - Community-based Q&A part of the Stack Exchange platform.
The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.
Super User - Super User, part of https://alternativeto.
Askalot - Askalot is a next generation CQA (Community Question and Answer) system.
AskBot - Askbot enables users to create Q&A forums.