(in no particular order)
www.bttf.com/futuregirl/index.html
One of the two best Fan Fiction sites around, both linked from bttf.com! Kristen Sheley has now 17 stories (and still going!) on display, plus other goodies like a profile page and a Novelization of an early draft of the Back to the Future screenplay.
http://www.bttf.com/futurevisions/
Watson's Fan Fiction
The second of two of the greatest Fan Fiction writers in the genre. 13 stories written by veteran fan, known as Watson.
http://www.oocities.org/Hollywood/Boulevard/8966/index.html
Christina Soberon's collection of Back to the Future fan fiction, inspired by Kristen. Collection includes piece from various writers including another ‘Back to the Beginning.’
http://www.fanfiction.net/master.cfm?action=story-listfiles&categoryID=176
A small number of Back to the Future Fan Fiction stories (compared to the hundreds of Star Wars stories). Among them is an origin story entitled ‘Before there was Time.’
http://home.att.net/~charlesmurphy10/bttf.html
Augie’s BTTF site. His fan fiction mixes smoothly with his love for the Beatles.