Q&A Websites: What are some more sites similar to Stack Overflow?

You should have a look at 'Pubble' - Irish based company - providing distributed Q&A solutions / messaging platforms which allows any organisation to engage with their online community direct from their our site. Core Q&A / Live Q&A / Twitter Q&A and Updates (which is a really cool tool similar to Chatter or Yammer). Check them out on Q&A Platform for Websites, Wordpress, Blogs & Facebook