http://meta.stackoverflow.com/questions/2267/stack-overflow-clones/37953#37953

Ive just looked into all the open source implementations mentioned, to summarise:

Most active that look like the best bets

• shapado Ruby, mongomapper and mongodb. Live site, also provide hosted sites for free. AGPLv3. Seems to be under active development as of June 2010.
• OSQA . Django. Live site. GPLv3. More actively developed fork of CNPROG.
• ASKBOT Django code, docs, import data. GPLv3. To install type "pip install askbot".
• Question2Answer. Live site PHP, MySQL. Actively developed. GPL license.
• CNPROG. Django. Live site(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.

Ones that look sort of finished

• openoverflow.
  Ruby, PostgreSQL, Haml. MIT license. No example site, and I cant find
  anyone using this one. Not a lot of recent activity; last code commit,
  May 2009 (as of Feb 2010).
• phpancake. PHP, Zend framework, MySQL. Live site. MIT license. Last activity Jan 2010. Renders very poorly in some browsers.
• cahoots. PHP, MySQL. GPL, MIT license. Officially inactive; demo site offline; last update Aug 2010.
• Qwench PHP. Example site (no pages except question list). No activity since Dec 2009.
• soclone. Django framework. MIT license. Seems to have gone stale in Nov 2008 (and only started on Google Code at the end of Oct 2008).
• Plurk Solace. Python. BSD license. No example site, and I cant find anyone using this one.
• stacked ASP.NET, Ra-Ajax, ActiveRecord ORM. GPLv3. Live site. Seems to have gone stale in Jun 2009. Some legal issues, but they seem to be over now.
• Qanda PHP, MySQL, MIT. Repository. Site inaccessible.

Others that seem to be work in progress

• FortyTwo. Python, Django, CouchDB. Example site. Unknown license. Little info.
• kerjakelompok. No example site. Unknown license. Little info.
• SmartR. No example site. Unknown license. Little info.
• Stack Underflow.
  C#. No example site. License is "do whatever you want with it" (I
  havent defined a formal license yet). Written as a learning project.
• T002_rails-overflow. Rails. No example site. Unknown license. Little info. Apache license.
• LampCMS PHP 5.3, MongoDB LGPL 3 License

Sorted by language

PHP

• Question2Answer
• cahoots
• phpancake
• LampCMS

Python

• ASKBOT
• CNPROG
• OSQA
• soclone
• Plurk Solace
• FortyTwo

Ruby

• Shapado
• openoverflow
• T002_rails-overflow

http://ASP.NET

• stacked http://ASP.NET

C#

• Stack Underflow

Unknown

• kerjakelompok
• SmartR

Did I miss any? Disagree? Please add a comment or update this answer.

And I dont really want to get into listing clones, but I have just come across Answer Bag which seems to be an all-encompassing clone covering everything from aviation through programming to religion. Or maybe that was there first? And Just Answer is another...