That’s the first I time about a fanfiction, but this one is certainly worth it. Its size and quality has been already recognized by many readers and has suscited more than 12000 reviews. The first chapters were published in February this year (2010). EDIT 2011/06/01: there are now 72 chapters.
It is an alternative history of Harry Potter, with a few initial different hypothesis who have far reaching consequences compared to the choices made by Mrs Rowling.
It is a very good mix of (among other things):
– Harry Potter (mainly the first three books) by you-know-who for the basic context and overall plot and characters
– The non-A world by van Vogt (explaining rationality and sanity)
– Ender’s Game (battle school and growing for an incoming battle) by Orson Scott Card and its sequels
– Dune (a few allusions to the first book, but especially The Emperor of Dune and its reflection on power)
As the author himself says, it is a good idea to have read at least the first volume of the original Harry Potter, but I would even says that you enjoy the alternative history even more if you have read at least to the third one (or seen the films).