|Here's an otter because everyone loves otters.|
Publisher's Marketplace - subscription-based, but probably the single best industry resource for publishing news and agent research. PM provides information on publishing deals (so you can find out which agents have sold books that are similar to yours or at least in your genre -- the database is searchable for the past 2 years) and sales data on published books. There used to be a free 1-month trial available, I don't know if this is still the case.
Predators & Editors - provides address listings for agents and editors (as well as resources for visual artists and musicians)
Query Tracker - provides a database of agents as well as a tracking system to track your queries.
Chuck Sambuchino's Guide to Literary Agents - maintained by Writer's Digest, provides a database as well as short posts with the wish lists of specific agents.
Agent Query - provides a database of literary agents with contact information and lets you search for agents by genre.
#mswl on Twitter - "manuscript wish list" tweets from agents that specify what type of work they are looking for right now. For example: "Quiet," literary MG that packs a serious emotional punch, like Crystal Chan's BIRD or Holly Goldberg Sloan's COUNTING BY 7s"
Query Shark - blog maintained by literary agent Janet Reid with hundreds of critiqued queries. She's very blunt about what works and what doesn't in a query. The blog archives are a fantastic resource for learning the do's and don't's of queries.
Pitch Wars and Pitch Madness -- periodic "festivals" of pitching on Twitter run by Brenda Drake. http://www.brenda-drake.com/
Pitch Madness (Pit Mad) happens every spring and lets writers submit a 35-word pitch (yes, 35 words only) and the first 250 words of their manuscript for consideration by a team of readers who select the best 60 manuscripts to be pitched to a slate of agents who then choose to request full or partial manuscripts.
Pitch Wars assembles a slate of published authors, editors and agents who choose writers to mentor through a revision of their work-in-progress. Writers are chosen similarly to Pit Mad, with writers pitching their novels on Twitter and mentors requesting to see more of it if they are interested. The mentors then choose one writer each with whom to work to polish the manuscript and get it to publication quality. In the second phase, the writers pitch the revised manuscript to participating agents and editors who request full or partial manuscripts if they're interested in the project.
More information about both can be found on Brenda Drake's site.