StackOverFlow.com is one of the best sites for your technical questions to be asked and answered! This has successfully taken the hat from the premium Experts-Exchange.com service as the best Q&A site!
Though StackOverFlow is around, many a times, we may need to have our own question n answer setup which is private to enterprise, or simply to have a similar site.
If you are looking to setup a similar site for questions and answers, you are in luck as there few competent open source & free Q&A engines that just does that job of SO!
A highly scalable Q&A engine written in PHP that uses MongoDB for hyper scalability. It has deep integration with social media such as FB, Twitter etc. Can be hosted easily on LAMP or XAMP environment.
Shapado is Q&A for enterprise or communities similar to StackOverFlow. Capable and easy to install. Though a premium service, Shapado offers a free plan with unlimited traffic with no custom ads and no custom domain.
Stacked is an open source implementation using .Net. Source code is also available for you to modify if needed.