What script is used to run hacker news?

HN runs on software written by pg (and others?), using a language called Arc (developed by pg and rtm).

http://ycombinator.com/arc/arc3.1.tar is newer than the arc2.tar referenced above.

You could have a look at http://pligg.com/ - it's an open source Digg-alike. I've used it on a company intranet and it's not bad.

It is surprisingly easy to set up

You might want to checkout the Reddit source - http://code.reddit.com/

I believe it is ARC Forum.