Score = (P-1) / (T+2)^G
I'm sure that YC-affiliated posts have a different ranking algorithm, considering you can't vote on them.
Some other cool algorithms are:
del.icio.us / delicious.com 's: ranked by upvotes in the last hour. I like the elegance of this solution a lot :)
reddit's "best" feature: compares the rates at which comments are upvoted to generate the probability that a comment is a good one, which is how posts with fewer votes can be ranked better than posts with more votes. It works well, in my opinion.
http://www.quora.com/Hacker-News/How-do-you-get-on-the-front...
There was a post back in 2010 with the formula which PG verified. It's probably changed slightly but not substantially since.