Is there a Hacker News back end open source alternative?

The current version of Arc Lisp (which includes the forum) can be found at http://arclanguage.org/

And the public, open source fork of Arc and the forum are at https://github.com/arclanguage/anarki

Bear in mind nothing publicly available is at feature parity with Hacker News because HN is essentially closed source for various (mostly YC business) reasons.

Here’s the original: https://github.com/wting/hackernews

Lobste.rs is one: https://github.com/lobsters/lobsters