Adventures in Agentland — blog by Natalie Lakosil (my agent, though that's not the reason why this is listed as a resource) with posts on querying, writing, and other publishing topics

AgentQuery — agent listings and querying tips

Nathan Bransford's blog — includes detailed posts on how to query agents and deal with publishers

Predators & Editors — list of literary agents and warnings if they have a known history of engaging in shady processes

Publishers Marketplace — info regarding agents and publishers, including sales histories

Publishers Weekly — publishing news

QueryTracker — website with agent and publisher listings that allows you to track who you've queried