A highly arbitrary and by no means thorough list of other stories or sites to check out, for the discerning BtVS fanfic reader (updated August 2005):

Peasant's Plot. Stories about the Fanged Four in the 19th century, focusing mainly on Spike, and often delightfully slashy. Peasant has a very confident command of the Victorian idiom and atmosphere, and is a wonderful yarn-spinner. 

Also known at Coquette, under which name she wrote in a Peasant-esque vein, the writer now known as Witling writes intelligent, heart-breaking slash stories, focusing on Xander/Spike, Wesley, and sometimes on the characters of Smallville. Her fics are never less than enthralling. Find them here. Her latest (as of August '05, is The Assistant, a dark S/X tale, seen through the eyes of Wesley, and featuring some incredible Angelus badness. One of the best darkfics out there. Heartbreaking. 

Deeply emotional stories (mostly slash, but with the occasional Spike/Dawn or other het pairing) told with admirable subtlety and restraint are the specialty of Wesleysgirl, whose site is here. 

Adversaries by Miranda.   This romantic and excitingly well-plotted S/B story introduces a couple of intriguing past slayers, who nearly steal the show. NC-17.  

Anna S's Buffy Season Noir Our heroes find themselves trapped in a Sunnydale cut off from the rest of the world after an invasion by an army of fascist demons bent on world domination. An alternate season of Buffy, dark and disturbing and bitterly romantic. Probably never to be finished, but the dozen or so "episodes" are well worth reading, each deep and rich and brilliantly populated by the whole cast of characters. There's S/B here, but it's by no means the sole focus of these ensemble tales. Anna is also the queen of the Spike/Xander ship—even if the idea of this makes you want to wash your eyes out with lye, take a deep breath and read "Sidelines," "Throwing Shapes," and most of all, "Subtleties.". More recently, Anna has taken a foray into Alias fic, writing a series of hot and psychologically plausible stories pairing Jack Bristow and Michael Vaughn. Even if you've never seen this show, these fics are worth pursuing. Find them collected here 

Margot's Femfic These are old school smut—old school in that they predate canon S/B, and old school in that they're often the sort of nonconsensual sex fantasies many women don't like to admit they occasionally enjoy. The prose can be a bit bumpy, and Margot could've edited things a bit for length, but this stuff is really filthy, in the best possible way. 

Rahirah's S/B Fic series—A multi-part novel-length epic, with an S/B pairing and use of the entire ensemble of BtVS and AtS characters. Terrific character development here, and plot that works. "A Raising" gives us a Buffy resurrection that's better than the official one! 

A Glorious Morning Have I Seen by Miss Murchison. In which Spike finally gets to make love to someone who has never been mean to him. There isn't a lot of Spike/Joyce out there ... this story makes me wonder why not.

Crossing Spike over into the world of Firefly, Justhuman's WIP The Outside Man & Serenity is full of surprises and pleasures, beautifully voiced, hot and funny. Link goes to her LJ site, where the parts of the story appear in reverse order, so scroll back to the beginning to read. 

Devilpiglet's Spike-fic is superfine, and is here.

Three classics of the S/B 'ship and all around rollicking fics with great plots and comedy touches are the Bowiehabarata stories by Mustang Sally and Rivkat.
    

Kalima recommends:

Fanfiction by Wisteria.
Spike/Buffy stories. Check out "98 Octane," "Happiness" and its sequel "Sweeter Than Oranges," and the new WIP (as of 7/02), "Ten Thousand."  

FayJay recommends:

Elena's Fiction.
Includes excellent post-The Gift sequence "After the Fall". Gutting stuff. Then there's the witty and pornalicious goodness of "Other Pervert" (Anya/Xander) and "What a Wicked Thing to Do" (Xander/Spike)  

Runaway Trains at 3am
by ChristinaK. Lovely (non porny!) Smallville/Buffy crossover. Probably the best portrayal of Dawn I've come across so far. Funny and sweet but not cloying. The sequel is Lines of Communication  

Rhymes with Gleaming,
by TheGreenChick. Beautiful little William piece that wraps snugly around the Siring scene in "Fool For Love."  

Comfortable by Shrift. Possibly my favourite Gunn/Wesley story so far.  

Naked by The Mad Poetess. (Oz/Lindsey.) Improbable idea, but lovely execution.  

If by some miracle you haven't happened across Yahtzee's writing yet, what the hell are you waiting for? Beautifully observed & plot-driven BtVS and AtS fiction, as well as such oddities as StarWars Slash that makes "The Phantom Menace" infinitely more palatable.  

Salieri recommends:

Two stand-outs of the Spike/Buffy pairing are Europanya and TheDeadlyHook.

The stories of Europanya —Sharp, visceral, vivid writing; amazing Spike voice; both funny and deeply moving.

The stories of TheDeadlyHook —Her fics are intelligent and show a deep level of thought and analysis. She has a knack for getting into the characters' heads, and her Spike/Buffy is wonderfully painful.