852 Prospect: The Sentinel Adult Fiction Index -- All Slash, all the time
Blair Angst Fiction Page -- The Sentinel. Gen-fic
The Complete Kingdom of Slash Archive -- Slash. All sorts of fandoms
Down in the Basement -- Archive of Slash fiction, with lots of great links
FanFiction.net -- Lots of Fan Fiction, all sorts of fandoms
I Made This Productions - X-Files Virtual Season Eight
RatB -- Mulder/Krycek Fiction
Strange Realities - X-Files Alternate Universe Archive
Susan Foster's GDP Series -- The Sentinel AU
The Written Word -- The Sentinel. Check out the Challenges Page