One of the resources that has been incredibly helpful in this process is Publishers Marketplace. I joined there about a year ago and have been enjoying reading the updates and deals news ever since. It's especially exciting when I see friends' books listed. I actually have been known to squeal out loud when I've seen their names--even if I already know about their deals.
The things that I have found most helpful lately? 1) The searches that show what agent reps what author or a particular book. It's been nice not to have to scour the house for the books I love (especially since some of them are lost in the black hole that is my daughter's room) and hope that the author gives a shout out to his/her agent. 2.) The individual agent pages that show what they are looking for with links to their own websites and what publishers they are working with mostly. 3.) The top dealmakers page has been really helpful too.
I could keep going. I've been up and down and around the PM site finding what I could. Not everyone updates their deals regularly, but that's a good thing to know too. It goes to personality.
Some of the other sites I've found particularly helpful are AgentQuery and QueryTracker, but there is no substitute for agent interviews in knowing what they are really looking for and what they are like. A special thanks to Cynthia Leitich Smith at Cynsations for all the marvelous interviews and Casey McCormick at Literary Rambles for all their fabulous agent info and links to interviews.