I'm looking for the open source/commercial engine for Question & Answers website like StackOverflow. Please recommend.
I should have the possibility to customize the engine with my own functionality If I'll need it.
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At the time, it seemed to work okay - definitely not as slick as Stack Overflow, but still worthwhile (q&a, comments, tags, votes, badges). Ultimately the culture of of contributing to Q&A didn't take off in company, but I participated for a while, and it was fit for purpose.
Looking at it now, it seems that the open source version gets very little love: it's still running Django 1.6(!!) and numerous PRs are pending (including an upgrade to Django 1.8 a year ago).
If you're a python person, then I'd recommend checking out the network graph / PRs. The basics are there, if you're looking for a 'fixer-upper'.
Talkyard is open source Question-&-Answers software. It's a cross between StackOverflow and novel forum software like Discourse and Flarum.
In addition to Q&A topics, there are Idea type topics — if your colleagues / customers / users have ideas about how to improve things, and would like to post feedback and discuss. There're open-ended discussions, e.g. for talking about news and blog posts.
Talkyard is open source, GitHub repo here, I'm developing it. There's optional SaaS hosting.
Below is a sample discussion, actually copied from this StackOverflow question: (CC-By-SA)