Methods Of Rationality (fanfiction)
Harry Potter and the Methods of Rationality is a fanfiction by "Less Wrong" (actually a pseudonym of Eliezer Yudkowsky) which postulates that the orphaned Harry Potter has been raised by a scientist stepfather and now has the equivalent rationality skill of roughly, oh, an 18-year-old Eliezer, i.e., massively flawed but still better than anyone else he's going to run into at Hogwarts.
It would be fair to understate that this fic has proven itself popular.
TV Tropes page here (warning, potential spoilers).
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