The Public Query Slushpile- this is a fantastic blog started by a fellow aspiring writer. People are able to post their queries and get advice about how to make them better. Some of the criticism is harsh (I know from personal experience), so make sure you have a thick skin and a real desire for feedback before you venture in. Also, if you post a query it's nice to leave feedback for others, too. (My first attempt and the query that got my agent's attention are both posted here)
Query Shark- I'm sure most of you know about this one. At Query Shark, agent Janet Reid gives query advice to people brave enough to have their query ripped to shreds. Just reading the queries that are already posted there is extremely informative. You can get a sense of the things that immediately annoy agents and also read several fantastic queries.
Charlotte Dillon's Website- several people mentioned this was a great resource yesterday and I agree. I loved the page with all of the winning query letters.
There are a lot of other blogs that focus on query writing. (The industry blogs listed on my sidebar all have wonderful posts about writing query letters.)
Searching For Agents:
Agent Query- This is a searchable database of agents. What I love is that you can search by agent, agency, clients or even book titles, so you can find out who represents your favorite authors.
Preditors and Editors- A fantastic resource for background checking agents. Do you want to know if an agent has sales? If they charge fees? If they are members of AAR? All of that information is available at Preditors and Editors. A Pink Recommended after an agent or an agency is a pretty good indication that an offer from that agency should make you very happy. A red Not Recommended should definitely make you think twice before querying. If there is neither a Recommended or a Not Recommended (this is the case for most of them) then you might want to do more research.
Bewares and Background Checks at Absolute Write- These are forums where writers can discuss their experiences with agents or agencies from query response times, to feedback (or lack thereof) from manuscript submissions, to what agents are like after they offer representation.
Publishers Marketplace- If you pay the $20 per month subscription fee you can search all of the deals made by agents (or at least the ones they reported) since 2000.
After You Query:
QueryTracker- This is mostly an organizational tool, but I found it very helpful. You can select agents you want to query and keep track of them after you query. Under the Reports and Statistics bar you can see the response times of individual agents as well as the genres they have requested pages from in the past.
I hope this wasn't all review (I'm sure it was for some of you). These are the websites and blogs that were most helpful to me during the query process. Do you have any favorites I missed?