Questions and answers are useful online resource centre that enable us to get quick, instant answers from community members.
This is an niche that hasnt really gotten that much attention or buzz in recent years, but over last 6 months seems to have gotten so much publicity.
I have decided to name a few of those Q&A sites have managed to deliver quick, insightful and useful answers, especially those that have an healthy community around them. They are a few new-comers.

The list are in no-particular order....

Quora

Definitely the hottest Q&A website at the moment, founded in June 2009, Quora aggregates questions and answers to many topics and allows users to collaborate on them. Quora was co-founded by Adam D'Angelo (Facebook's former CTO) and Charlie Cheever. Hopefully, it doesn't turn into a website with spammy answers. There's a lot of really rich, high-quality content on there, with answers from investors, key entrepreneurs...basically most of the top people on the web that you wish would be answering your questions.

Mahalo answers

Mahalo answers is another community driven Q&A platform, launched on 15th december 2008. Mahalo Answers allows questioners to give a monetary reward (called a "tip" on the site) to the user who provides the most helpful response.

Aardvark

Aardvark is a Q&A chat service that tries find people to answer your questions among your friends, friends of friends and people who know something about your subject. I havent really seen any real growth on there since they were by google early last year. However, the real strength of Aardvark is in the instant messaging and mobile access options.

Yahoo answers

I'm sure everyone is probably familar with yahoo answers, it is amongst the most established and used Q&A websites since its launch in 2005. You can ask questions, answer other people’s questions, find out what other people are asking. You may also add cutesy avatars to represent your in the Yahoo Answers community. A free question and answer community.

Wikianswers

Wikianswers was founded by jimmy wales, founder of wikipedia, it uses the same formula and technology that made Wikipedia a successul open encylopedia. You can immediately ask questions or look for answers to question already posted on the site using category browsing or topics browsing.

Wiki answers

Wiki answers is a mini-answers.com site with a wiki twist. the site allows anyone to participate and contribute to the development of the question and answer site. Wiki Answers lets you answer questions, post your own question, search for questions asked by other members and more.

Answer bags

Answerbag tries to break down questions into categories, you can ask questions on any topic. You can also browse through questions in selected categories of your choice and read all the questions and answers posted by other people.

Fun advice

FunAdvice.com is a social networking question-and-answer (Q&A) website. The website does a good job of integrating questions, answers and photo in a exciting way for users to interact. According to Quantcast traffic statistic, In December 2009, The website gets 3.1M visits per Month in the US.

Ask ville

Askville is a user-driven research site owned by Amazon.com. Askville is a place where you can share and discuss knowledge with other people by asking and answering questions on any topic. After asking a unique question, the answers get sent to you via email which enables you to go through the answers.

Form spring

Another new-comer to the list, Form spring is a bit different, The site allows its users to set up a profile page, from which anyone can ask them questions. The questions and their given responses are then published on the user's profile page. The questions can be asked with a user's name hidden, or they can be visibly sent from another Formspring account, Users can however choose to disallow anonymous questions, and have the ability to block selected people from asking further questions, even if the asker has remained anonymous.