The first pieces I wrote were just short vignettes inspired by gameplay; they're in the
Game Stories {link} thread.
Later, I had a solo game with an interesting starting position, so I started writing a history of events. That .txt file is up to 11K now, but it's coming out really dry, so I'll need to edit it a bit before I put it on public display.
I've got another one planned, where I've already written a scene after the First Contact, with placeholders for the race involved and alternate Versions for friendly or hostile contact. I've also got a separate file for that one with a list of all the characters I've used, to keep track of who they are as the story develops.