http://jetreidliterary.blogspot.com is a blog by Janet Reid. Her description says
" I am a literary agent in NYC. I specialize in crime fiction and narrative non-fiction (history and biography.) I'll be glad to receive a query letter from you; guidelines to help you decide if I'm looking for what you write are below. There are several posts labelled "query pitfalls" and "annoy me" that may help you avoid some common mistakes when querying."
Amongst the resources on her blog are her query letter checklist, query letter FAQ, and query letter diagnostics (why you didn't hear back!).
(update- note that since this resource was originally posted Janet has moved to New Leaf Literary http://newleafliterary.com/ however she will still answer querries sent through her blog)
Janet Reid, Literary Agent
A blog by the literary agent Janet Reid, on writing-related topics