These two by the same author are probably my favorite post-"Born Free" reveries so far:
Damned If You Do...Damned, Period (Dex/Deb, NC-17)
It All Comes Down to Blood (Dex/Brian), R) Even if you're not into the Broyay, it's worth reading for the dynamic between them. (The whoo-ha is implied, not explicit.)
Remember the VO in one of the early eps where Dex says he dreams of a post-apocalyptic world without people? This is on a site devoted to end-of-the-world scenarios from all fandoms:
Tiny Monsters (Gen, no spoilers, Dex, Deb)
These are all dry, dark fics that do a great job of getting across the characters' voices.
dexter_fanfic is an LJ comm for, well, Dexter fanfic of all genres. It's the only Dexter fic site I know of so far.