Here's another good author I recently found at fanfiction.net, with several excellent Silmarillion-based stories (although one or two are slashy) -
Finch. Also, check out this story by Joan Milligan -
Alqualonde- and these two by Le Chat Noir,
The Star in the Ocean and
Of Iron and of Steel. And check out their favorite author and story recommendations (linked under their author pages) - the folks who write good stuff generally seem to recommend other good pieces, and it helps avoid the fruitless searching through endless Mary Sue and "I love Legolas" stories that are cluttering up fanfiction.net these days. People ARE writing good stuff, it's just hard to find!
I second the suggestion to read Mithadan's stuff here at Barrowdowns, also. It's a pity that he hasn't written more (HINT, HINT!).