Yahoo! Answers!

Quora.com is a question/answer service allowing users to pose questions that other users can answer, each question and answer can have comments, each users can write one individual answer and there can be a summary answer.Each user can choose a description explaining their area of expertise or questions and answers can be anonymous.Questions and answers are searchable.

Stack Exchange is a fast-growing network of 84 [and counting] question and answer sites on diverse topics from software programming to cooking to photography and gaming. We are an expert knowledge exchange: a place where physics researchers can ask each other about quantum entanglement, computer programmers can ask about JavaScript date formats, and photographers can share knowledge about taking great pictures in the snow.

Slant is a collaboratively edited resource that helps you quickly make decisions.

Super User, part of Stack Exchange , is a Q&A site for computer enthusiasts that's free. Free to ask questions, free to answer questions, free to read, free to index, built with plain old HTML, no fake rot13 text on the home page, no scammy google-cloaking tactics, no salespeople, no JavaScript windows dropping down in front of the answer asking for $12.95 to go away. You can register if you want to collect karma and win valuable flair that will appear next to your name, but otherwise, it's just free. And fast. Very, very fast. »

Server Fault is a Q&A site for system administrators and IT professionals that's free. Free to ask questions, free to answer questions, free to read, free to index, built with plain old HTML, no fake rot13 text on the home page, no scammy google-cloaking tactics, no salespeople, no JavaScript windows dropping down in front of the answer asking for $12.95 to go away. You can register if you want to collect karma and win valuable flair that will appear next to your name, but otherwise, it's just free. And fast. Very, very fast. »

Startups get your queries answered by the World's Top Investors and Entrepreneurs. Startups + Investors +Entrepreneurs Connect.

Bitfortip offers a unique use case thanks to Bitcoin.

OSQA is the free, open source Q&A system you've been waiting for. Your OSQA site is more than just an FAQ page, it is a full-featured Q&A community. Users earn points and badges for useful participation, and everyone in the community wins.

Scoold is inspired by StackOverflow and implements most of its features. You can ask and answer questions, upvote and downvote posts, earn badges and reputation. Each edit creates an new revision, making it easy to follow the history of edits. Scoold is optimized for the cloud. You can simply click the 'Deploy to Heroku' button and get up-and-running in minutes. You can deploy it to your favorite PaaS, too.

Curositi is a caring and supporting community of people who are eager to learn things. It provides an environment that offers opportunities to communicate new knowledge and solve problems through collaboration with experts and other members of curositi.Curositi helps individuals to engage in a meaningful inquiry based learning that has genuine value. We are encouraging individuals to invest in learning things which they truly care about and make it a habit, rather than viewing it as an activity forced upon them. »

AllAnswered.com allows any community to create question and answer boards for any topics that they are interested in. It is absolutely free to use no matter how big or small your community is. AllAnswered.com strives to be the place where people can get their questions answered and build a strong community by helping each other.

Fitness Chirp Rooms are free instant user created chat rooms that are run by fitness professionals and community members. It is completely free and helps both general community members by having their questions answered quickly and fitness professionals by giving them an easy way to promote themselves and their name. Not only can you have your own questions answered but previous answers are searchable... so your question could already be answered. If not start your own room or learn about fitness from other users questions now! »