Ask HN: Is there an open source social news platform?

I know of one that's extremely similar: http://www.arclanguage.org

Scoop, http://www.scoopdev.org/, was fashionable for a while. I've not been involved technically, so I cannot say whether it counts as mature http://vkpedia.com/touchline/doc/scoop_admin_guide/1_3_Syste... or neglected http://www.scoopdev.org/story/2008/1/22/43049/0753

Reddit's open source, as is Slash (the code behind Slashdot).

pligg is a good one.