Quora

ArrayShift

A question/answer site built to emulate the core functionality of the StackOverflow site and the StackExchange platform. It currently implements a reasonable subset of the site’s functionality, but is incomplete and far from bug-free. It has rough edges that need to be smoothed out and functionality that needs to be finished in other contrib modules before this package could be considered a full feature-clone of Stack Overflow or Quora.

http://drupal.org/project/arrayshift
Free
http://i.imgur.com/TBNVGyy.png
PHPancake

This system purpose is to create a community site, where decisions, content and the face of the site can be managed in a kind of a Starship-troopers democracy (the more you donate to the community, the more privileges you have). The plan is to create a complete community content based site, which will include the the wanted mix of categories, tree-forums, linear-forums, article/tutorials submissions (and blog entries fall under that too).

http://sourceforge.net/projects/phpancake/files/
Free
http://i.imgur.com/TBNVGyy.png
Solace

Solace is an open sourced knowledge exchange platform similar to Quora. Written in Python with extensibility in mind, Solace builds upon the work of several existing collaboratively edited knowledge exchange and support services. Solace incorporates existing elements such as up/down voting, topic trending/relevancy algorithms, a progressive reputation and reward system, tiered permission levels, user moderation, and wiki like editing to create a powerful self-sustaining platform that stays relevant and ensures users easily find what they are looking for.

http://opensource.plurk.com/Solace/
Free
http://i.imgur.com/TBNVGyy.png