|Harry Potter and the Wastelands of Time|
Read and available freely online.
Harry Potter fan-fiction, often mentioned by fans of Harry Potter and the Methods of Rationality. Published periodically, presumably, but not sure over what time period. Possibly began in 2008. Last updated in 2010 anyway.
From the FanFiction.net copy. 282609 words according to FanFiction.net, 1497859 characters according to Notepad++, which is about 272338 words, which at 350 words per page is 778 pages.
Eliezer Yudkowsky has mentioned this as one of his own favourite fanfics. As of July 2012 I've had it bookmarked for at least a year I think. I started it abortively once, but if it wasn't for reading Harry's Game and Brutal Harry recently and finding that I seem to quite enjoy Harry Potter fanfic I think I'd have let this one pass. I will probably give it another try soon.