(This list was last updated: 16th April '07 to update an entry with a working link and a new word count.)
Shyarra made a list of really prolific HP fanfic authors some time ago, but as it is no longer hosted on the internet, I thought I would attempt to recreate it from the original thread on FictionAlley. This is the first themed list I've ever assembled, so bare with me while I iron out the wrinkles.
Please note: (These aren't rules, really. I just thought I'd terrify you all by letting you see how my mind works, even if only in relation to this list. If you've got something interesting, drop me a line and we'll see where it fits.)
If you see something erroneous, be it title, author, word count, or a blatent error in the summary, please let me know. I will fix the problem as soon as possible. (The summaries are all my doing unless otherwise noted. Finding full summaries was a bit of a pain, so I made up some random stuff. Consider yourselves lucky (or unlucky, as the case may be) I did more than list the title and author. If you want to know more, follow the links.)
Fics over 2,000,000 words:
I hesitate to post this, simply because I am in awe. In compiling this list, I found myself re-totaling The Sacrifices Arc and here's what I have:
The Sacrifices Arc by Lightening on the Wave Word total 2,611,027 and counting. (Harry's twin brother Conner is thought to be the Boy-Who-Lived. Harry, meanwhile, has been charged by their mother to protect Conner at the cost of all else.)
*shrugs* I still haven't read it, but there it is. On with the rest of the list.
Completed fics of 1,000,000 words or more:
Barb's Psychic Serpent Trilogy Word total 1,321,037 for all three fics. 1,466,616 if you include the prequel to the trilogy. (Harry Potter gets a snake that can see the future and lots of hijinks and shipping ensues.)
Semprini's Trilogy Word total 1,093,533 for all three fics. (Harry Potter learns to use the Energy of Love to defend against Unforgiveable curses, then teaches others to do so.)
Several fics by Kimberjingle are said to be longer than a million words each, but I am not familiar with any specific works, so I am including him/her in this list warily. Most are Harry/Draco and are NC-17.
Incomplete fics and series 1,000,000 or more:
The Danger Series by whydoyouneedtoknow Word total 1,129,769 and counting. Thanks to zjiht for the total. (An OC with the unlikely name of Danger take Harry into her care giving him a better life... And so on and so forth. I haven't actually read this one either.)
Incomplete fics of 500,000 words or more:
Updated! 4-16-07 The Alchemist's Cell and The Heart of Gryffindor Trilogy by SJR0301 Over 926,325 words. Thanks to Oomahey for fic information. (Another one I haven't read. Authors Summary syas: Post OotP Harry runs from the Dursleys, only to find he can't enter the wizarding world..) It's been found! Thanks to the anonymous poster who found the above link. I copied and pasted into Word Perfect for the word count, if anyone's got a different answer, let me know. When complete, this might pass a million.
Preflight and Flight of the Thestrals by GD-7 Word total 898,840 and counting. (A Draco/Ginny fic and it's prequel. Oddly enough, both are WIP.)
The Hero Trilogy by joe6991 Word total 852,462 and counting. (I have no idea what this one is about. I'm waiting for the author to finish it. Author's summary is really long and sounds angsty.)
Added 29-10-06 - Before Harry: The Marauder's Tale by Chelles Word total 811,511 and counting. Thanks AussieVampiress (Third part of the trilogy is a WIP. It starts with MWPP and Lilly's first year at Hogwarts and ends... We'll have to see, I guess.)
Bobmin's Sunset Over Britain and Sunrise Over Britain Word total 784,968 and counting. Thanks AussieVampiress for the word count. (Harry/Hermione fic with independent Harry and Manipulative but misguided Dumbledore.)
A Year Like None Other by Aspen in the Sunlight Word total 756,194 with a WIP sequal. (After a letter from home brings Harry's family problems to light, Snape takes an interest in his well-being.)
Resonance by Salamander (GreenGecko on FF.N) is 484,149 words and I have hopes that it's WIP sequel will reach similar lengths, putting it at or near the million mark. (Snape adopts Harry after the Defeat of Voldemort. Follows Harry through his final year at Hogwarts and through Auror training.)
The Seer's Truth Series: A Broken Beginning & Beyond the Darkness by Lady Lestrange Word total 795,175 Thanks zjiht! (I know nothing about this fic.)
In the Crosshairs by Dragon Voldemort Word total 679,975 and counting (I know nothing about this fic.)
The Legacy of Two Generations by LadyoftheLake Word total 661,693 and counting. (A Lily/James fic, I don't know more than that.)
Midnight Guardian series by ksomm814 Word total 616,073 and counting. (Harry is adopted by Sirius and Remus in third year.)
Mastermind Hunting by Louis IX Word total 586,880 and counting. (I haven't read this fic, but it seems to include a very smart Harry running from a manipulative Dumbledore.)
Added 29-10-06 The Paradigm of Uncertainty Series by Lori Word total 557,899 and counting. (I read PoU ages ago, but I didn't know there were sequels. The third part is a WIP. Post Hogwarts with Hermione and Harry as the main characters.)
Added 29-10-06 The Silent Seige and sequals by swishandflick Word total 555,560 and coutning. (I haven't read this one. From the author's summary, Harry fakes his death when Voldemort attacks him in LIttle Whinging.)
Loss of Innocence and Rise of the Phoenix by Ioci Word total 543,162 (Post OotP, despite that it looks interesting. This pair barely sneaked in.)
Added 29-10-06 Harry Potter and the Fifth Element by Bexis Wortd total 541,380 and counting. (Harry's summer and sixth year with Harry/Hermione pairing. Author likes initials a whole lot, so I'm having trouble understanding his summary, but it might be interesting.)
The Birthday Present by excessivelyperky Word total 532,772 and counting. (I know nothing about this fic.)
Harry Potter and the Veil of Darkness by Oceans Phoenix Word total 517,648 and counting. (6th year fic, actually looks fairly interesting. Might be superpower!Harry.)
The Of Readings and Randomness series by gryffindor12 Word total 514,618 and counting, (MST, sort of. Lily and the Maruaders get copies of the HP books and read them together. Might top a million if the author does all seven.)
An Alternate Adventure: The Goblet of Fire by slytherin-nette Word total 510,791 and counting. (Sequel to a yr 1 fic in which Harry and Draco became friends instead of enemies on the train.)
Completed fics at or over 500,000 words:
The Refiner's Fire and The Time of Destiny by Abraxen Word total 983524. (After OotP, Harry spends the summer pulling himself back together with the help of a muggle family he befriends. Has Harry/OC)
The Draco Trilogy by Cassandra Claire Word total 896,762 (Harry and Draco end up in each other's bodies semi-permanently after using a botched Polyjuice potion in class, only no one else knows.)
Eclipse by Mayfair Parts: 1 2 3 4 5 6 7 Word total over 600,000 according to Oomahey. (Post OotP, RL/OC in Beginning and H/G later. I haven't read it because you have to sign up on the boards and I am simultaneously too lazy and not despearate enough for something new.) You have to sign up to read this one. If anyone knows someplace else it's posted, let me know.
The Book of Morgan Le Fey & The Final Reckoning by LavenderBrown Word total 619,214 (A sixth-year adventure told from Ron's perspective.) Author's summary.
After the End by Arabella and Zsenya Word total approximatly 590,000 Thanks to RaisinGirl for that number. (It's the summer after seventh year, and Harry, Ron, Hermione, Ginny, and the rest of the wizarding world must learn to live without fear.) Author's summary.
Per Solem Lacuna: By Words Alone by wintermoon2 Word total 571289 and counting. (Enchanted diaries bring solace to unlikely lost souls. It's a wizarding twist on the classic tale of penpals. What will happen when these mystery writers finally meet? HD) Author's summary.
New Beginnings: Teecie's Story by Foodie Word total 576,797 (A long fic in a series of much shorter works. Apparently OC-centric.)
That's everything I've got. If you know any more, let me know and I'll add them. Also, if a WIP changes category, say so and I'll move it.
Enjoy your fics!