I actually built one back in the day: http://akkartik.name/post/2011-05-13-05-47-29-soc. Trouble was, it kept getting banned/blocked even when I scaled back to crawl HN every five minutes. Eventually I was crawling so infrequently that I was missing stories that dropped off /newest in the meantime, so I stopped crawling.
I actually had an arc version for a brief period, but it didn't seem like there was as much need for it here. But perhaps the email notifications would be useful. I'll see if I can bring it back.
 I was certain I'd showed it here, but no I only showed it over email to a couple of folks. I really should bring it back just so y'all can look at it. Or at least locate the (rails) sources.