Since I'm in the process of querying, I thought I would share some of the things that I am using to target my agents.
Querytracker.net - Wonderful source of information, including links to a lot of resources and a way to track who you have and have not queried. Also has a great forum. I especially like the quick query button where it makes it easy to send out email queries and it even keeps the query on file so you know what query you used on them.
Agentquery.com - Another resource for agent information. Has details about their previous deals, as well as what they are looking for now.
Facebook - If the agent has a profile, you can usually use their status updates to figure out what sort of project they are looking for right now. Not always, but some agencies also have their own Facebook pages.
Twitter - Love twitter just because of its informality. You get some good personal info on agents if you follow them and as an added bonus get to make connections as well.
Google - Just searching for an agent's name can usually pull up recent interviews for them allowing you to get an idea of current trends and a sense of them as people.
The agent's website - The BEST source for information on recent submission guidelines and details about their policy on whether or not they reply to all queries or if no reply means "no."
These are the main tools that I'm using. What do you do when researching agents? Am I missing any resources that I should be using?