This thread hasn't been updated in a while, but I haven't properly posted in a while, so I guess that works out well. Right?
I haven't read much Tolkien-related fanfiction in a while, either, but one author whose works I continue to follow is
AuriMynonys. This is because after all these years, I'm still pulling for Grima and Eowyn, can you believe it? What can I say, I'm a sucker for doomed romances. She's one of the best authors for the pairing, to be sure. Most of her fics fall in the PG to PG-13 range, but one or two are more "adult," so just use your judgement.
(And thank goodness some of those fics on the 'Downs archive are gone. I think I had some in there, and my high school fanfiction is much better left lost to the ages...)