According to Wikipedia, A Q&A website is a website where the site creators use the images of pop culture icons to answer input from the site’s visitors, usually in question / answer format.
All blogs and websites need backlinks to get higher ranking in search engines. Question and Answer sites or Q and A sites make it easy for you to get high PR backlinks quickly. High PR backlinks to your site will help your site to get higher pagerank. If you want to rank well in search engines, then you will need quality backlinks with quality content.
Hope, This list will help you improve search engine ranking. Do let me know via the comments if I have missed any Q & A site which you think should have been included in this list and also tell me which are your favorite Question and Answer sites from this list and why?