I enthusiastically second the recs for all the fics mentioned, and especially those by knifeedge, anaross, mustangsally/rivkat, darkapple, holly, herself and unbridledbrunette. Besides those already rec'd here, I'd add the following extremely well-written fics:
Clocks of the Long Now by lostboy (here on TSR)
Summary: This story begins post-Chosen, then rockets into an angsty, dystopian post-NFA future that finds Buffy questioning the loyalties of her closest allies.
Four years after the gang helped Sunnydale perform its swan dive into Hell, a terrifying new enemy emerges from an unexpected quarter. As secrets are revealed and old friends are reunited, the greatest Apocalypse of all looms ever closer. Will they manage to save the day one last time?
Adieu, A Dew, I Do by LizerrrbeathanSummary: AU/ seven years post 'Chosen' --Willem is in hiding in Newcastle upon the Tyne, UK; given the magnet of hot souls and mutual minds--he is found.
Haunted by kantayraSummary: AU after Dead Things. Seven years ago, Spike fled Sunnydale and his abusive relationship with Buffy. Now, he and still-best-friend Dawn are working together in their own demon hunting agency. However, when they're called out to investigate a chain of supernatural murders at Cascade Mountain Lodge, they discover that Buffy's on the case as well. Will they be able to solve the mystery before they become the next casualties? And what does the solution have to do with the events of seven years ago?
100 Years of Solitude by IsabelleSummary: Series set 100 yrs after the Season five finale. This is part AU and futuristic. Spike has been alive for 100 hundred years after Buffy's death. Whistler comes to him to let him know that Buffy has been summoned to fight a new evil.
The Bittersweets series by HerselfSummary: She can't go on, she'll go on. Spike and Buffy meet again, post Wrecked.
Cousin Arabella by lilachigh (here on TSR)
Summary: A party, lots of demons, hatred, passion, white hot jealousy and several glasses of pink champagne. Not a good mixture for a Slayer and a Vampire whose cousin is trying to make Buffy admit her feelings.....
Journeys Series by Mary (series is a WIP)Summary: We are shaped and fashioned by what we love...
Promise of Frost by EurydiceSummary: When a snowstorm strands Buffy and Spike, they are forced to turn to each other for strength, especially when a missing Giles, mysterious visitors, and way too much magic threatens to turn it into a not so very merry Christmas.
Spiegel Im Spiegel by fallowdoeSummary: Buffy never makes it up Glory's tower. The experiences of Spike and Buffy in an apocalyptic world.
Prophecy and Warmth by TheBear (here on TSR)
Summary: She tries not to think of him, she is afraid if she does he will consume her and there will be nothing left of her for the daytime
Where Pies Go When They Die by ghostyouknow27Summary: Hell, as it turns out, serves a great cherry pie.
Heroes in Hell by shapinglightSummary: Post-Chosen. The events of AtS season 5 are playing out but without Spike's involvement, for reasons that will become clear pretty quickly. There are also very vague references to BtVS season 8, the comic. However, familiarity with it is not needed to understand the story.