QueryTracker is the best resource for querying writers, hands down.
It tracks your queries, lists agents and publishers, what they represent, and if they're open to queries. You cannot go wrong with using this site.
Writer's Digest is another great site because it spotlights agents and provides querying tips and successful queries.
Query Shark is great because it not only critiques queries, but it has archives of them to help out new querying writers. Just reading them for reference can be helpful.
Absolute Write has a section called Query Letter Hell, and is the largest community for it to date.
Agent Query Connect is also a forum to post a query for feedback.