I'm surprised there were so few answers to this when originally posted. Nice lists so far, though.
I can't propose a definitive list, but off the top of my head I'd be choosing from,
William Hope Hodgson: "The Voice in the Night"
Fritz Lieber: "Smoke Ghost"; "The Sunken Land"
Karl Edward Wagner: "Undertow"; "River of Night's Dreaming"
Poppy Z. Brite: "His Mouth Will Taste of Wormwood"
Caitlin Kiernan: "Le Peau Verte"
C. A. Smith: "The Empire of the Necromancers"; "Xeethra"
Thomas Ligotti: "The Frolic"; "Les Fleurs"; "Teattro Grotesco"
Fred Chappell: "Lineus Forgets"
Walter de la Mare: "Seaton's Aunt"
Guy de Maupassant: "The Horla"
Ambrose Bierce: "The Damned Thing"
Kathe Koja: "The Neglected Garden"; "Angels in Love"
John Langan: "On Skua Island"
Jonathan Carroll: "The Panic Hand"; "The Sadness of Detail"
Robert Aickman: "The Inner Room"; "The Hospice"; "Ringing the Changes"
Algernon Blackwood: "The Willows"; "The Wendigo"; "Ancient Sorceries"
Arthur Machen: "The Great God Pan"; "The White People"
M. John Harrison: "The Great God Pan"
I'm pretty sure I could add as many more if I consulted my books at home -- I think I'd find something by Gerald Kersh, for instance.
Afterthought: Sheesh. A Lovecraft forum and I forgot to add Lovecraft:
"The Colour Out of Space"
"The Rats in the Walls"
"The Dreams in the Witch-House"
Randy M.
