[ http://meta.stackoverflow.com/questions/2267/stack-overflow-clones ]
Most active that look like the best bets
 . Java.
. AnswerHub is a full-featured enterprise Q&A platform built by the people who created
. AnswerHub is actively used by eBay, LinkedIn, GE, NetApp, Epic Games, Unity3D, Havok Games and many more leading companies.
. GPLv3. To install type "pip install askbot".
 PHP 5.3, MongoDB LGPL 3 License
 PHP, MySQL.
Not quite so active
 (currently 404) WordPress Plugin. Actively developed by a company that looks reliable. No longer free; $19 to download (with no membership)
 Django, hosting, actively developed, non-free
 Has a demo, is free, looks a bit different then stack overflow and is missing some features. It is a little more related to reddit. It is in development still and they are taking suggestions for features, themes, and modules. You can request this stuff and fix bugs in the demo. Make sure to check it out
. Has hosting, is powered by PHP and MySQL, and is customizable and user friendly to the owner and users.
 SaaS, free 30 day trial. Haydle is focused on being a private, internal Q&A system, so has some different features that make sense when used internally like teams, etc.
Ones that look sort of finished
 Ruby, mongomapper and mongodb.
, also provide hosted sites for a fee. AGPLv3. Seems to be under active development.
 . Django.
. GPLv3. More actively developed fork of CNPROG. OSQA is in maintenance mode, with security and Django platform updates, but few new features.
 has replaced OSQA as DZone's primary Q&A solution.
. 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.
. PHP, Zend framework, MySQL.
. MIT license. Last activity Jan 2010. Renders very poorly in some browsers.
. PHP, MySQL. GPL, MIT license. Officially inactive; demo site offline; last update Aug 2010.
. PHP, MySQL. MIT license.
 (no pages except question list).
No activity since Dec 2009
. Django framework. MIT license. Seems to have
gone stale in Nov 2008
 (and only started on Google Code at the end of Oct 2008).
. Python. BSD license. No example site, and I can't find anyone using this one.
Last commit was Dec 2009
 ASP.NET, Ra-Ajax, ActiveRecord ORM. GPLv3.
Others that seem to be work in progress
. Python, Django, CouchDB.
link leads to a 404. Unknown license. Little info. [Last commit in January 2011]
. No example site. Unknown license. Little info.
. No example site. MIT License. Little info.
. 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.
. Rails. No example site. Unknown license. Little info. Apache license.
(Update 21/01/2011: link leads to a page which says CNPROG is officially closed). GPLv3. Not a lot of recent activity; last code commit, Oct 2009 (as of Feb 2010). Primarily developed in Chinese, although there are English translations.
No longer supported according to the website. Was used as base by OSQA & Askbot
. Python, Flask, and Couchbase. GNU GPLv3 or later. Documentation is
Sorted by language
Did I miss any? Disagree? Please add a comment or update this answer.
And I don't really want to get into listing clones, but I have just come across
 which seems to be an all-encompassing clone covering everything from
. Or maybe that was there first? And
 is another...
Note: AnswerHub was formally Qato.
I might as well throw my two cents in. I've developed a site from scratch which uses the Stack Overflow model, but it is based on a completely different subject matter (
Super Street Fighter 4
- There is an
 which allows for users to search on events related to the game (as well as vote up/down)
- There is a
 (again, podcasts and episodes can be voted up/down)
- There is a
. Notes differ from questions in that they are one-way projections, there are no replies. Almost like blog entries, but on a specific topic that's indexed all the same.
And there are plans around other forms of data around the game as well.
I've asked a number of questions about (and on, not directly related to) Stack Overflow to help shape functionality:
All of these were used to help shape the development of the site.
 (Russian: ХэшКод) - Russian clone of SO.
Also there are
 (РутКод) and
 (БитКод) related projects, which clone SF and SU respectively.
Doctype launched recently, they a have a similar concept to Stack Overflow.
But the visual design focuses too much on the person and not enough on how good an answer is…
DocType closed on 2013-02-14
, and they didn't bother to release their codebase.
I wouldn't call my
 an SO clone, but it is inspired by it. I got tired of the noise that is phpBB. I am starting to feel the pain now, I need a meta site.
Stack Overflow clone in PHP -- "Qwench"
Piskvor's Semifinite Monkeys
 - not open-source, apparently clones the SE 1.0 model as well (hosting SaaS Q&A sites). It took me a while to notice why the
DynDNS community support pages
 felt familiar.
Polish version of StackOverflow. It's look like SO year or 2 years ago. The site looks like author don't update anything on page. Even years on foot are out date.
Like many others, I build
a Stack Overflow clone
 as well, mostly as a learning exercise.
This one might be the only open-source clone written in C# /
My open source project of a Stack Overflow clone is
hosted on GitHub
It's under heavy development. And will add more functions compared to Stack Overflow.
Cybermaxs - Betclic
 (Social Q&A Community)
Seems fairly similar, I don't know if it infringes directly. See the
 I posted on this site.
I managed to get
 (a PHP Stack Overflow clone) from inscripts / anantgarg working under IIS 5.1 - with
Extending to provide search and additional adminstration functions to manage Q&A should make the application more useful.
 is a Drupal project that replicates a lot of the SO functionality. It's currently in Alpha.
 seems like a new attempt, no risk for SO though...
It's in production, but anemic, and prone to spam questions and answers. There's probably a better chance to get bada answers at SO proper.
 Complete with question title, original formatting, images, and profile pics...