1. QueryTracker: An incredible resource where you can search agents who represent YOUR genre so you don't waste time querying non-fiction agents with YA Fantasy. You can also check out whether those agents are accepting queries and how to find out their submission requirements. You can also use QueryTracker to find lists of the most queried agents and those that are least responsive. You can even keep track right on the site and read comments about others experiences. The best news? It's free. You can upgrade, however, for even more.
2. Predators and Editors: Want to find out if an agent you just heard about is on the up & up? Want to see if there have been any past issues reported? That's exactly what you should and can look up on this site. It will either give you peace of mind or something to ask about if you get to the phone call stage.
3. The Writer's Digest New Agency Alerts: A great way to see spotlights/interviews with new agents in the industry.
4. Literary Rambles: A great writing blog with lots of agent spotlights to check out. You can read interviews and also find links to other sites where you can connect with them. Stalk the agents! They want to know you did your homework.
5. Publisher's Marketplace: I know that a membership is costly, but it may be worth it. Either way, you can get info on agents, agencies, and recent sales by browsing the list at this link.