I like the concept of online Q&A sites and I have to say that I've been quite impressed at how successful Yahoo! has been with Yahoo! Answers. Not only did they build a good end user experience but they followed up with heavy cross promotion on their other services, TV ads and getting lots of real-world celebrities to use the service. My favorite questions asked by real-world celebrities thus far

Based on your own family's experience, what do you think we should do to improve health care in America? asked by Hillary Clinton (U.S. Senator and Presidential Candidate)

What should we do to free our planet from terrorism? asked by Dr. APJ Abdul Kalam (President of India)

That's pretty freaking cool. Kudos to the Yahoo! Answers for being able to pull off such a great promotion and build such a successful service in such a short time.