There's always Lamer News, "an HN style social news site written in Ruby/Sinatra/Redis/JQuery"
I recently came across Coordino (http://coordino.com/). I haven't tried it out myself, but it looks pretty polished.
It's also Open Source.
Openqa is a clone of stack exchange for django. There are several. Google is your friend here.
nice. test out your hypothesis first. :)
try OSQA. Heard it's good. thanks. get a host like uhm webfaction.
Goodluck! Outsource to RoRers / pythonistas in the Philippines.
I heard this guy is good - bryanbibat.net
HN itself is open source (included with Arc: http://arclanguage.org/install), as is reddit (https://github.com/reddit/reddit/wiki). It does sound like Openqa is a better option though.