This one is called Preditors and Editors, and it's possibly the most valuable resource available to writers trying to get published. It contains a HUGE database of publishers and agents, listed alphabetically, with updated contact info. Even better, it helps writers distinguish between the legit agencies/publishers and the not-so-legit agencies/publishers. A very important distinction!
Before I ever send off a query letter to an agent, I go straight to this site, to double-check the quality of the agency first. To see if the agency has any "red flags," or even if the agency has recently gone out of business. Many times, after checking with this database, I change my mind about submitting to that particular agency, and I've probably saved myself some potential grief in the process. As well, I'm able to see positive remarks about the good agencies, and my hopes for those agencies are reinforced.
It always pays off to do your research, and this particular site should be an essential part of any writer's agent/publisher research process.