Here are the classical formulas: http://www.seomoz.org/blog/reddit-stumbleupon-delicious-and-hacker-news-algorithms-exposed
Formula:
(p - 1) / (t + 2)^1.5
Description:
Votes divided by age factor
p = votes (points) from users.
t = time since submission in hours.p is subtracted by 1 to negate submitters vote.
age factor is (time since submission in hours plus two) to the power of 1.5.