Fan fiction is fiction written about the Battlestar Galactica universe. There are extensive amounts of fiction written about both the original series as well as the re-imagined series.
Re-imagined Series Fan Fiction
Battlestar Gemicon - collaborative fan fiction based on the Scifi.com Messageboard
Original Series Fan Fiction
"The Best of Battlestar Galactica Fan Fiction" - collection of links to TOS fiction
BattleStar Gemicon is a hybred work of fan fiction and live play by post and chat role playing game. It has a General Master (Rachel Faith Anderson) who started the game. It has over 30 writers, a dozen of which post daily. It runs one "game" or Episode weekly during the off air season. It is currently on Episode 3 of its second season, having completed 12 episodes in season 1.