Query Shark
Query Shark is a blog that teaches you to revise your query letter so that publishers will sit up and beg. And really, who doesn't want to see a publisher with their little paws up in the air?
The way it works is that you send in your query letter (after reading their guidelines here) and Query Shark will edit your entry as a teaching post. Be forewarned, though, you MUST follow their guidelines or you won't stand a chance.
Even if you aren't currently in the querying phase of things, it's a great site to check out for future reference.