Question2Answer - A Q&A site helps an online community to share knowledge.
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.
Blurtit - Blurtit is a Q&A website. You can ask a question or answer on any question.