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.
Agents and editors are very active on twitter, and post what was right and wrong with queries they received.
The #500queries and #tenqueries are worth scrolling through to get an idea of what can go wrong.
Agent advice is also very useful and can be found in the #askagent and the #QueryTip tags.
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.
#MSWL is a fantastic compilation of twitter tags of agents and editors looking for certain works. If you find something similar to yours, add MSWL in your query subject!
Manuscript Wish List is another site dedicated to the wish lists of agents and editors.
How do I know pitch contests work? I'm one of their success stories! And you can be too.
This pitch contest calendar by Clarissa Taylor has every upcoming contest and links to their rules. She also has a great pitch generator you can use. So hone your pitches and get those queries ready!
Goodreads has several groups in their community section to connect beta readers, as does Absolute Write.
Reddit's Destructive Readers is also a good community to get feedback. Or you can check out their general writing subreddit to look for beta readers.