Free Sites

• Yahoo! Answers - The largest of all the sites by far, Yahoo! Answers stole the crown from Google by creating a free question and answer community.
• WikiAnswers - A Q&A community based around a wiki. Anyone can answer a question, then anyone else can come along and edit the answer to refine it.
• Askville - Amazon's Yahoo! Answers clone.
• NowNow - Another Amazon Q&A site: ask a question and get three answers by email. Though free during beta, NowNow uses Amazon's Mechanical Turk web service to pay real people for answers. They anticipate costs in the $0.25/question range.
• Yedda - Standard free question and answer site.
• Lycos IQ - A Yahoo! Answers clone from Lycos UK that also incorporates social bookmarking.
• Live QnA - Microsoft's Yahoo! Answers clone.
• LinkedIn Answers - Focused on business questions.
• Trulia Voices - Real estate Q&A.
• Minti Questions and Answers - This one is all about parenting.
• Wondir - Similar to Yahoo! Answers.
• Rediff QnA - A question and answer site from Indian portal Rediff.
• Fluther - One of the newer sites. Fluther supposedly learns the type of questions you like to answer as you use the site and directs more relevant queries your way.
• Wis.dm - Another fairly new site, Wis.dm focuses on yes/no questions.
• Answerbag - Similar to Yahoo! Answers, Answerbag tries to break down questions into social (fun), expert (serious), local, and shopping areas.
• Oyogi - Oyogi had a lot of promise when it first appeared, but now seems to be a ghost town.
• Say-so - Say-so lets you give people a choice of answers (like this).
• Grupthink - Not questions and answers, but 'open-ended topics' and 'opinions' at Grupthink. (It's like Yahoo! Answers with thumbnail photos and comments on each answer.)
• Dizzay - A very basic Q&A site.
• KnowBrainers - A question an answer site with a MySpace-like social network attached.
• The Answer Bank - A simple answer-based community from Britain.
• PointAsk - Standard Q&A fare.
• AllExperts - Ask questions directly to volunteer experts.
• Able2Know - A question and answer system built on top of a forum (looks like it started out as phpBB).
• AskMe Helpdesk - Ask volunteer experts and then tip them with cash if you get an answer you like (and you're feeling charitable).
• What Should I Say? - Work and relationship advice seem to be the most prevalent topics at What Should I Say?
• Simply Explained - Similar to Yahoo! Answers, the more you participate, the higher your "expert rating" can go.
• Ask a Librarian - Send questions by email to a network of librarians using this service from the US Library of Congress.
• Askeet! - Askeet! is a near direct clone of Yahoo! Answers, but it's also the demo project for the Symfony PHP framework. So if you want the source code for your very own Yahoo! Answers clone, then look no further.

Pay Sites

• Uclue - Uclue is almost identical to the old Google Answers. In fact, it was started by ex-Google Answers researchers. Ask a question, set a price, get an answer.
• JustAnswer - More or less the same thing as Uclue, though perhaps in a prettier package.
• Kasamba - Live chat with paid experts who charge their own price by the minute.
• Ask.MetaFilter - In order to cut down on spam, MetaFilter's Q&A site charges a one-time $5 registration fee.
• ExpertBee - Experts bid on your question at ExpertBee and you pick the one that gets to answer.
• Experts Exchange - Ask questions directly to experts; this site charges a monthly fee to participate.

Other Sites

• ChaCha - Experts guide you on your web search and help you find the results you're looking for (or, that's the idea).
• Ether - Ether lets people set themselves up as paid phone consultants. My guess is that at some point they might set up a directly of consultants using their service.
• Qunu - Talk directly to experts via a Jabber IM client.
• Everyday Questions - The idea here was to take questions and answers and turn them into funny flash cartoons. Doesn't seem to have happened, but that hasn't stopped people from continuing to ask questions (if not actually answering them).

Conclusion

There are so many question and answer sites out there that one company actually does marketing services specifically targeted at these sites. I was actually very surprised by how many of the above web sites are quite active. Personally, Uclue is the only one I have any meaningful experience with -- and I was satisfied with the results I got for my $10. Which of these sites would you recommend? Did I leave any off the list? Let us know in the comments.