Stack Overflow - Community-based Q&A part of the Stack Exchange platform.
Stack Exchange - Stack Exchange is a fast-growing network of 84 [and counting] question and answer sites on diverse...
The Answer Bank - The Answer Bank is the UK’s leading question-and-answer site.
AskBot - Askbot enables users to create Q&A forums.
Askalot - Askalot is a next generation CQA (Community Question and Answer) system.
Scoold - An open-source, enterprise-friendly Q&A platform inspired by Stack Overflow.