On platforms
I’m starting with a list from this (famous) Meta.SE thread. I’m only considering the ones marked as open-source:
https://meta.stackexchange.com/questions/2267/are-there-any-clones-alternatives-for-running-a-stack-exchange-style-qa-site
Askbot
Askbot doesn’t offer a demo version (right now?), so I can’t comment on its features. However it is build using Python+Django, a framework that was not only unanimously voted against but is also using a dynamic language which doesn’t scale well. (Edit: seems to have a base system similar to SE)
Biostar
Despite being open source, it seems to be built for a secific purpose instead of a general platform. Same technology concerns apply.
Django-knowledge
Is no longer maintained, seems to be more a support center, same tech concerns.
LampCMS
Same tech concerns apply, cannot comment on features, as demo doesn’t work
Mamute
Seems quite dead to me (not sure though), demo sites not working, so cannot do feature analysis
PaizaQA
Uses a tech stack voted against, demo site doesn’t work properly (login failure with given demo accounts)
Kliqqi-CMS
Is dead, its technology was also voted against (IIRC without proponent votes)
Question2Answer
Seems quite good, however has tech stack issues as others above. Not sure about moderation aspects
Scoold
Basic user management, moderation, image upload features are not open source
Talkyard
Uses tech stack voted against, is more of a forum than a Q&A site
Vanilla forums
Tech stack concerns apply here too, also seems to be more of a forum
On tech stack
The current choice of tech stack was done, because of concerns of scalability and type safety. I’ll ping @Marc.2377 for this, as he is the tech lead.
On this post
I’m making this post wiki (editable by everyone), so that we can update this list without adding more answers.