There are many Q&A sites nowadays. Many of them are extremely successful while others have failed (Who remembers Google Answers?).
Among the most popular are Yahoo Answers and the pioneer Answerbag.com. Others like Quora.com are gaining success rapidly while sites like Answers.com get an estimate traffic of around 90 million unique visits a month.
Most of the Q&A sites share many features but they have notable differences in design and usability too.
Which one is your favourite?
You can see a list of Q&A sites here.
You can also read a good article interviewing the Q&A site CEOs here.