Harry Potter and the Methods of Rationality

Series: Harry Potter
Characters: All of them
Genre: Hard Sci Fi
Summary: This is not a strict single-point-of-departure fic - there exists a primary point of departure, at some point in the past, but also other alterations. The best term I've heard for this fic is "parallel universe".
Status: Ongoing?

Written by Yudkowsky. This guy's a genius.