I've found that source code of http://stackoverflow.com is not open-source. What similar apps are there that have source code available?
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They practically "forked" the initial version of SO and from there created a Django based version which is open source.
Saying it was "forked" is a decent way to say they copied everything that was public on SO site and make it theirs ( how is that called in english?? .. oohh yeap.. stoo... stol. stolll.. oh I forgot )
There is one that was created called Knowledge Exchange. There was another called 'Stacked' but their page appears to be down, so I don't know the status of that project.