Hacker News Reader

It was 500 when I hit it, but occasionally goes up. Not sure what it is now.

I hit it at 500 a few weeks ago. Ok, not sure exactly but definitely not more than 510 :)

What algorithm are they using? Is the source open?

HN is in fact open source (an older version of it anyway): http://arclanguage.org/

However much of the "secret sauce" is, well, secret.