I’m starting with a list from this (famous) Meta.SE thread. I’m only considering the ones marked as open-source:
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)
Despite being open source, it seems to be built for a secific purpose instead of a general platform. Same technology concerns apply.
Is no longer maintained, seems to be more a support center, same tech concerns.
Same tech concerns apply, cannot comment on features, as demo doesn’t work
Seems quite dead to me (not sure though), demo sites not working, so cannot do feature analysis
Uses a tech stack voted against, demo site doesn’t work properly (login failure with given demo accounts)
Is dead, its technology was also voted against (IIRC without proponent votes)
Seems quite good, however has tech stack issues as others above. Not sure about moderation aspects
Basic user management, moderation, image upload features are not open source
Uses tech stack voted against, is more of a forum than a Q&A site
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.