Turn Away by Simon Field is a stunning little Jayne vignette that answers the question of what Jayne did during the War.
Passengers by Harriet Vane does an amazing job of mapping the crew to the players in The Odyssey, Metamorphoses and The Iliad in this nifty plot-y little story involving Serenity getting Shanghai'd by the Alliance into transporting a Prefect's family to a colony. While her grammar and punctuation skills can be rough, her ear for dialogue and gift for characterisation and plot make it worth it for me.
The Fair by maystone is a lovely little vignette exploring both Kaylee and Inara's friendship, and Inara and Mal's prickly courtship. There are a few technical errors in the way of POV shifts, but they don't detract much, and there's a lovely wistful feel to this piece. Added bonus: Jayne as a sloppy affectionate drunk.
Refuge by Lizbet is a thoughtful and interesting plot-driven story. I'm not 100% sure I buy the tech (felt a bit more B5 or Farscape than Firefly), but Lizbet's plot is strong, and she shows a deft hand for capturing the characters' voices.
The Founders of the Feast by Grey Bard is a lovely slice-of-life story set before Simon, Book, and River joined the crew. Love Grey's Kaylee and Wash in particular.
The Uses of Temptation by maystone is a gorgeous Book story. You'll never look at the preacher quite the same way again.
Wiretap by Ana takes a very different look at Bushwhacked by way of a war memory involving Mal and a young beau named Declan. Ana's storytelling style is simple and effective and evokes a lovely sense of grace.