West of the Moon, East of the Sun by knifeedgefic (NC17, Ensemble) Over 400,000 words, complete.
On LJ / On BSV / On SR / On EF (members only) / On FF.net
Strap in for an entirely alternate S5 from the POV of the characters. It features dreamscape rendevous, silencing spells, dimension hopping, tall towers, and very bad weather, among other things.
Clocks of the Long Now by lostboy_lj (R, Ensemble) 200,000 words, complete.
On LJ / On DW / On SR / On EF (members only)
Four years after the gang helped Sunnydale perform its swan dive into Hell, Buffy's in Rome, Riley's in the Army, Kennedy's in Vlad's castle, Dawn's in a bunker, Giles is in deep cover, Spike's wearing something very odd indeed, and nobody really knows where Willow is. It might be up to Xander and Andrew to save the day. A fascinating look at good and evil from every perspective.
The Barbverse by BarbC rahirah (NC17 in places, Ensemble) Approaching 1 million words, some bits in the middle still in progress.
On AO3 / On Author's Site
Spans 50 years and explores the question of whether or not one Buffy Anne Summers, vampire slayer extraordinaire, and one William the Bloody, vampire of infinite heart and limited ethics, can reach a certain degree of mutual accommodation after diverse discourses and considerable ass-kicking.
Bittersweets by herself_nyc_fic (NC17, Ensemble) 400,000 words, complete.
On EF (members only) / On AAS / On Author's Site
After the events of "Wrecked", Spike urges Buffy to find out exactly why the chip no longer sees her as human. The discovery leads to a centuries long adventure, complete with time travel, dimension hopping, and even a trip in outer space.
Lost in Time by spikeskat (NC17, with mostly OC Ensemble) 65+ chapters, complete.
Only available via Wayback Machine, here: https://web.archive.org/web/201202281513
Ethan's S2 "Halloween" spell goes awry, and Buffy is sent back in time with Spike(!?!) as her bodyguard, courtesy of the Powers that Be. He's not a very nice vampire, either. At least, not until Buffy works a little magic of her own.