Free Speculative Fiction Online 
Isaac Asimov: Nightfall
(Analog (Astounding), September 1941)
Isaac Asimov: The Last Question
(SF Quarterly, November 1956)
. Alternate link.
Paolo Bacigalupi: The People of Sand and Slag (F & SF, February 2004)
Isfdb:
Neal Barrett, Jr.: Perpetuity Blues (Asimov's, May 1987)
Stephen Baxter: The Gravity Mine (Asimov's, April 2000).
Terry Bisson: England Underway (Omni, July 1993)
Kent Brewster: In the Pound, Near Breaktime (Tomorrow SF, October 1995)
David Brin: The Giving Plague (Interzone #23, Spring 1988). Alternate link.
Ted Chiang: Division by Zero (Full Spectrum Anthology #3, 1991)
Tony Daniel: Life on the Moon (Asimov's, April 1995).
Colin P. Davies: Tall Tales on the Iron Horse (Spectrum SF #8, March 2002)
Andy Duncan: Fortitude (Realms of Fantasy, June 1999)
Greg Egan: Oracle (Asimov's, July 2000) Isfdb:
Greg Egan: The Planck Dive (Asimov's, February 1998)
Michael F. Flynn: The Clapping Hands of God (Analog, July 2004). Isfdb:
Jeffrey Ford: The Empire of Ice Cream (SciFiction, February 2003)
. Available as part of this magazine.
Karen Joy Fowler: Standing Room Only (Asimov's, August 1997).
Neil Gaiman: How to Talk to Girls at Parties (Fragile Things Collection, 2006)
James Alan Gardner: The Ray-Gun: A Love Story (Asimov's, February 2008). Isfdb:
Tom Godwin: The Cold Equations (Analog (Astounding), August 1954).
Dominic Green: The Clockwork Atom Bomb (Interzone #198, May 2005). Isfdb:
Daryl Gregory: Second Person, Present Tense (Asimov's, September 2005) Isfdb:
Joe Haldeman: None So Blind (Asimov's, November 1994).
Joe Haldeman: Four Short Novels (F & SF, October 2003) Isfdb:
Robert A. Heinlein: The Menace from Earth (F & SF, August 1957). Also available on a freely distributable CD.
Kij Johnson: 26 Monkeys, Also the Abyss (Asimov's, July 2008). Alternate link. Isfdb:
Naomi Kritzer: The Year Without Sunshine (Uncanny Magazine #55, November 2023)
Yoon Ha Lee: Ghostweight (Clarkesworld #52, January 2011). 
Isfdb:
Yoon Ha Lee: Effigy Nights (Clarkesworld #76, January 2013) Isfdb:
Yoon Ha Lee: The Bonedrake’s Penance (Beneath Ceaseless Skies #143, March 2014). Isfdb:
Yoon Ha Lee: The Knight of Chains, the Deuce of Stars (Lightspeed, August 2013) Isfdb:
D. D. Levine: The Tale of the Golden Eagle (F & SF, June 2003) Isfdb:
Kelly Link: Flying Lessons (Asimov's, October 1996)
Kelly Link: The Faery Handbag (The Faery Reel Anthology, 2004)
Kelly Link: The Summer People (Steampunk! Anthology, 2011)
Kelly Link: The Specialist’s Hat (Event Horizon, November 1998).
Kelly Link: Carnation, Lily, Lily, Rose (Fence, 1998)
Kelly Link: Travels with the Snow Queen (LCRW #1, 1996).
Kelly Link: Most of My Friends Are Two-Thirds Water
David I. Masson: Traveller’s Rest (New Worlds, September 1965)
Ward Moore: Bring the Jubilee (Straus & Young, 1953)
Suzanne Palmer: Bots of the Lost Ark (Clarkesworld #177, June 2021) Isfdb:
Daniel Pearlman: The Heart of the Overchild (Real, Winter 1993)
Kim Stanley Robinson: The Lucky Strike (Universe Anthology #14, 1984).
Kim Stanley Robinson: A History of the Twentieth Century, with Illustrations (Asimov's, April 1991)
Benjamin Rosenbaum: Start the Clock (F & SF, August 2004). Isfdb:
Benjamin Rosenbaum: Biographical Notes to ‘A Discourse on the Nature of Causality, with Air-Planes’ by Benjamin Rosenbaum (All Star Zeppelin Adventure Stories Anthology, 2004)
Joanna Russ: When It Changed (Again, Dangerous Visions Anthology, 1972)
Eric Frank Russell: And Then There Were None (Analog (Astounding), June 1951)
Bob Shaw: Light of Other Days (Analog, August 1966). Also available on a freely distributable CD.
Lucius Shepard: The Jaguar Hunter (F & SF, May 1985)
William Shunn: The Practical Ramifications of Interstellar Packet Loss (SF Age, September 1998)
Cordwainer Smith: The Game of Rat and Dragon (Galaxy, October 1955)
Norman Spinrad: Carcinoma Angels (Dangerous Visions Anthology, 1967).
Michael Swanwick: The Very Pulse of the Machine (Asimov's, February 1998).
James Tiptree, Jr.: And I Awoke and Found Me Here on the Cold Hill’s Side (F & SF, March 1972)
Catherynne M. Valente: The Difference Between Love and Time (Someone in Time: Tales of Time-Crossed Romance Anthology, 2022)
Greg van Eekhout: In the Late December (Strange Horizons, December 2003).
Jack Vance: The Moon Moth (Galaxy, August 1961).
John Varley: Air Raid (Asimov's, Spring 1977)
Ray Vukcevich: Whisper (F & SF, January 2001) Isfdb:
Peter Watts: Blindsight (Tor, 2006)
Connie Willis: Fire Watch (Asimov's, February 1982)
George Zebrowski: The Word Sweep (F & SF, August 1979)