I just read Sideways (202,533 words) by GoldenUsagi and really enjoyed it:
http://spikeluver.com/SpuffyRealm/viewstory.php?sid=28394Alternate reality. Buffy starts as a normal college girl. But her life is slowly turned upside down beginning with the day she's unexpectedly in danger. Hired by chance to protect her is Spike, a vampire with a reputation for doing business with anyone for enough money. She's less than thrilled, but finds herself drawn to him even as she realizes he's more than he seems. But humans aren't the only ones after her, and when she's called as the Slayer, her whole world changes. Things with Spike, who she's discovered some realities about, are no longer as simple as they seemed. But being sent to the Hellmouth and realizing the high death rate of her calling, Buffy hires Spike as backup with the only thing she has left—Slayer blood. Winner of 20 awards!
Eurydice has a number of great, long stories that can be found here:
http://dark-solace.org/elysian/viewuser.php?uid=58My favorites are Black Satin Voices (205116 words) and Charms of the Clarion (222806 words).
Also, Herself's Bittersweets series is enormous and wonderful, but very dark in parts.
http://www.echonyc.com/~stax/Buffy/herself/index.htmSERIES: "Bittersweets": An ongoing series of Spike/Buffy tales, meant to be read in order. The first springs from "Wrecked" in season six; the series veers off from canon at that point. These stories are het NC-17, with some slashy flashbacks. Winner of Best Series in the 2002 Watching You awards. Nominated in numerous categories in the Fall, 2003 Eternal Devotion Awards. Winner Best Conventional Pairing Series, Outstanding Conventional Author, Round 8 of the Shades of Grey Awards, November 2003.