[ http://meta.stackoverflow.com/questions/2267/stack-overflow-clones ]
Most active that look like the best bets
. Enterprise Q&A by the people who created
. Not free.
: commercial Wordpress Plugin
: Java. Enterprise Q&A developed and supported by
. Not free.
. PHP 5.3, MongoDB LGPL 3 License
. PHP, MySQL.
. 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
. PHP, MySQL. Looks a bit different to Stack Overflow. Has hosting.
: SaaS, free 30 day trial. Haydle is focused on being a private, internal Q&A system.
Ones that look sort of finished
: Ruby, mongomapper and mongodb.
, also provide hosted sites for a fee. AGPLv3. Seems to be under active development.
. 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.
: Python/Django. Project officially closed. 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...
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.
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.
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.
Stack Overflow clone in PHP -- "Qwench"
 - 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...
There is also
community QA site
. It is clearly based on the SE model in its design, style, functionality, etc. It's content is a mix of Stack Overflow programming questions and Cross Validated machine learning & data visualization questions, with perhaps bits of other SE sites thrown in. Their self description is:
Where scientists ask and answer questions on machine learning, natural language processing, artificial intelligence, text analysis, information retrieval, search, data mining, statistical modeling, and data visualization!