I have over 1000 fan fics converted to RTF and with Calibre now able to create mobipocket formatted ebooks I can't recommend the program enough for anyone that has a Kindle, Sony PRS, or Cybook. it's a great program for file conversion and management.

Email me at amalthia at gmail.com and I can send you some direct links to my backup directory. It's not perfect because it's really easier to download everything at once, but my directory is organized by fandom, then author name and title.

there is a LJ community called http://community.livejournal.com/kindle_fanfic/profile?style=mine

I'm co-moderator for another fan fic ebook community. http://community.livejournal.com/fanfic_ebooks/profile?style=mine

The kindle ebook community may be what you're looking for the one I'm co-moderating is mostly information and tips and tricks and I share my personal collection there.

And there is the Ebook Library

The archive has over 800 stories archived there...