Since the source code for this site isn't available for purchase or external use, I'm looking for software that can run a Q&A environment similar to Stack Exchange. Are there any such pieces of software available?
Stack Exchange has released an updated, feature-complete version of the Stack Overflow engine which is now available for enterprise use. You can read more about that here
(Note that this is different from Stack Overflow for Teams, which is a hosted solution.)
But if that does not meet your needs, there are several third-party tools that try to clone at least some of the functionality here.
Most active that look like the best bets
Kliqqi: PHP, MySQL. Different look, hosting. Not active, CC-BY 3.0 license
Not open source
AllAnswered: Django. Include Q&A in its knowledge management system. Not Free.
Haydle: SaaS, free 30 day trial. Haydle is focused on being a private, internal Q&A system.
Rootbuzz: Django, hosting, actively developed, non-free
SabaiDiscuss: commercial WordPress Plugin
Programlama: Python, flask (In Turkish)
Unity Answers: Allowed topics are the Unity editor and C# (the site is free and in English)
Ones that look sort of finished
shapado: Ruby, mongomapper and MongoDB. AGPLv3. Website unreachable, Last commit in 2012.
openoverflow: Ruby, PostgreSQL, Haml. MIT license. No example site, and I can't find anyone using this one. Not a lot of recent activity; last code commit, May 2009.
phpancake: PHP, Zend framework, MySQL. Live site. MIT license. Last activity Jan 2013. Renders very poorly in some browsers; the live site states "I am aware of this site not working properly. I am working on it!".
cahoots: PHP, MySQL. GPL, MIT license. Officially inactive; demo site offline; last update Aug 2010.
Solace: Inactive; last commit in 2010.
Kliqqi: PHP, MySQL. Different look, hosting available. Not active, CC-BY 3.0 license
stacked: ASP.NET, Ra-Ajax, ActiveRecord ORM. GPLv3.
Arrayshift Drupal plugin GPLv2, last commit in 2016.
Others that seem to be work in progress
Asking: Perl, Mojolicious. Perl License (Artistic). Little documentation at this point. GitHub repository README.md in Portuguese.
kerjakelompok: No example site. Unknown license. Little info.
SmartR: No example site. MIT License. Little info.
Stack Underflow: C#. No example site. License is "do whatever you want with it" (I haven't defined a formal license yet). Written as a learning project.
T002_rails-overflow: Ruby on Rails. No example site. Unknown license. Little info. Apache license.
CNPROG: Python/Django.Project officially closed. Was used as base by OSQA & Askbot.
Sorted by language
- Q&A (Is a WordPress Plugin)
- Vanilla Forums
- openoverflow (last commit in 2009)
- T002_rails-overflow (last commit in 2009)
SmartRCreator announced it is deprecated.
- QARoR (barely any progress)
- Discourse (active, primarily a discussion forum)
MEAN (MongoDB, Express, AngularJS, and Node.js)
Did I miss any? Disagree? Please add a comment or update this answer.