Here’s a breakdown of the Flash fan fiction I have found. If you know of anything I’ve missed, please send me the URL.
- FanFiction.Net has a Flash category with about a dozen stories, some set in the mainstream DCU.
- Justice League Universe (JLU 2001) is a shared universe branching off after JLA #4. JLU 2001: The Flash has been written by Bill Kte’pi, John Rabon, and now Robert Rock.
- DC Infinity is a shared universe branching off after Infinite Crisis with an alternate take on “One Year Later.” It includes a Flash series by D. Golightly.
- Faux DC features fanfic in the present-day DCU. Faux DC: The Flash has been written by Bryan Albright, Doc Skorupa, and Jason Reeps.
- DC2 is set in a world very similar to the DCU, and features a Flash series.
- The Five Earths Project follows the five universes that survived Crisis on Infinite Earths, and imagines what they might be like if they had remained separate instead of merging into a single universe. You can find stories featuring Wally West, Barry Allen and Jay Garrick, including a collection of Earth-1 Flash stories.
- The DC Archive includes a Flash series starring Wally West and Jesse Quick.
- Louise Freeman Davis’ Thicker than Water explores why Wally failed as a mentor to Impulse. In Who, Disguised As... Bart needs to come up with a Halloween costume.
- Victoria Bishop’s First Impressions details the first meeting between then–Kid Flash and Robin
- Jim Greeno’s Amongst Friends explores the friendship between Wally West and Dick Grayson.
- Propaganda’s Bedlam Boys (and Girls) is a light-hearted tale of Impulse and the Star-Spangled Kid teaming up against White Lightning
These stories are set in the world of the 1990 Flash TV series.
- Amanda Ohlin’s Two of a Kind follows up on the TV series, focusing on Barry and Tina’s future son.
- The Lois & Clark Fanfic Archive has a Flash crossover by Kyle Harper. “The Flash” is written as part of The Krypton Club’s Unaired Fifth Season
I imagine there are probably several Smallville stories featuring that show’s Bart Allen, but I have not had time to look for them.
These stories are set in the world of the Justice League animated series and Justice League Unlimited.
- Teh_no has a series of Flash stories set (more or less) in the JLU universe. A bit off-color, but not explicit.
- FanFiction.Net has a Flash category with about a dozen stories, some set in alternate universes.
- DC/Marvel The Merging features a Flash series in a combined universe. Writers include J.M. de Joya, Bob Young, Chip Caroon and Black Condor, and John Phillips.
- DC Heroes features a Flash series by Bill Kte’pi focusing on the future Barry West.
- Ron Schablotski’s Barry Me Not is a western-era Speed Demon story.
- In Joe Grunenwald’s “Swift Vengeance” Elseworld, Bruce Wayne becomes the Flash. (Disappeared when AOL shut down Hometowns in October 2008.)
- DarkMark’s Funeral for a Flash focuses on the days after Barry Allen’s death in a multiverse that survived the Crisis, and Wally West’s first attempt to fill his mentor’s boots.
- Darth Yoshi’s A World to Leave Far Behind features that lost traveler from Hypertime, Walter West.
- Multiverse features several projects with Flash series:
- Legends is a continuation of the Legends of the DCU project, “building the DCU from the ground up.” Legends of the Flash focuses on a young Barry Allen and his former college professor, Eobard Thawne, who has spent his life developing his Speed Force theory.
- Earth-8 is set on the world briefly seen in Infinite Crisis, home to Kyle Rayner as Green Lantern, Jason Rusch as Firestorm, etc. Flash: Earth 8 features John Fox.
- Beyond is set in the Batman Beyond universe. Flash Beyond features Bart Allen’s son Reese learning the ropes as Kid Flash.
- Ultimate Flash is based on the “Ultimate” treatment given to the Flash by Wizard Magazine in 2002.
She was the leading expert in what made speedsters speed. She learned that all she thought she knew was wrong. The hard way. Now she knows the truth. She is the truth. The others rode the Lightning. Dr. Alia Devin became a part of The Lightning. She became “The Flash.”
DC Futures was a collaborative fanfic universe chronicling a possible dystopian future in the early 22nd Century. The writing was divided up the way the actual DC universe is: each author worked on a particular title. The Flash (written by Tommy Hancock) focuses on the above-mentioned Alia Devin, who sometimes faces off against the corrupt Flash Corps (an elite police unit made of of surgically-altered officers given super-speed).
*Digitallymystic.com went offline in October 2004, taking DC Futures with it. So far, I have found no indication that the site has come up again at a new location. At best, you can check out the Internet Archive copy of DC Futures.
Legacies never die. A legacy set by one can change hands to another. Sometimes, it is with slightly unexpected results. This is a story of a very unexpected result. An arrogant man who can “steal” speed from objects wants to be a hero. This is his story. The story of the Flash, hero of Keystone, the Two-In-One-City.
DC Future Shock was a collaborative fan-fic set in a more utopian vision of the 22nd Century. This time around, the speedsters are Allen sisters Jasmine and Trisha, known as Lady Windrunner and Quickstep. The twist: this era’s Flash, Quinn LeCorrant, can’t use speed—he can only steal it. The trio faced Mayor Brenton Thawne, Kadabra, new Rogues, and the return of Savitar.
Three chapters in, the writer of their Flash series decided to discontinue it and pulled it down from the site.
Digitallymystic.com went offline in October 2004, taking DC Future Shock with it. The site had stopped updating in March 2003, and no one has (to my knowledge) brought it online anywhere else.
Not exactly fanfic, but the Legion of Superheroes Elseworlds RPG has an original speedster character from the 22nd Century, a descendant of the Allen bloodline who bounced off the Speed Force and landed in the 30th Century.