Stack Exchange!

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.

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

Steemit gives users monetary rewards for any meaningful contribution to the platform. Of these contributions the most important are:- Creating or posting content which the other users consider valuable (like)- Finding valuable content and upvoting it

A Q&A site helps an online community to share knowledge. No matter what your area of interest, people with questions can get super answers fast. Question2Answer is a popular Q&A solution.

Askbot is a Q&A system, like StackOverflow, Yahoo Answers and some others. Askbot is based on code of CNPROG, originally created by Mike Chen and Sailing Cai and some code written for OSQA.

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

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.

Yahoo! Answers is a way to find and share information. You can ask questions on any topic, get answers from real people, and share your insights and experience.

Bitfortip offers a unique use case thanks to Bitcoin.

Solace is a fully open-sourced multilingual support and knowledge exchange platform written in Python.

Coordino allows you to create a question and answer system for you and your users to enjoy.

Grow your audience with a custom news community. Users can signup, submit links, like them, and comment. Style the look and feel of your community to match your brand. Use any domain or subdomain you wish for your community. Free and easy.

Shapado is a questions and answers site for your enterprise or community, your “knowledge base on the cloud”. It makes sure questions never get asked twice by making the information flow.

Mamut is a tool that allows to ask questions and give answers and comment on both. In addition users could vote for questions and answers and mark solved questions as such.