AgentQuery
www.agentquery.com
List of agents, the site appears to be choosey about who they include - reputable agents only.
Agent Research & Evaluation News
www.agentresearch.com/news.html
Articles on agents and deals.
Agent Research & Evaluation Verification Service
www.agentresearch.com/agent_ver.html
You can check out an agent's reputation here - as well as sales he/she has made, things like that.
Association of Authors' Representatives
www.aar-online.org
AAR is the professional trade group for agents in the US. The site has a membership list, and the AAR Code of Ethics is published on the site. Good to know.
Preditors & Editors
www.anotherealm.com/prededitors
Online site listing evildoing agents and their schemes.
Publisher's Lunch
www.publishersmarketplace.com/lunch/free
Free version - nice, but the subscription does provide so much more. For free, you get a daily email providing publishing news; once a week, the email gives you a listing of recent deals. Get the subscription from Publishers' Marketplace.
Publishers Marketplace
www.publishersmarketplace.com
Great site. Deals in an organized manner, going back several years. Agents can be researched here. It may not have all the scoop, but it does have a lot.
Publishers Weekly
www.publishersweekly.com
Online version of the magazine. You have to subscribe. Expensive.
Writer Beware
www.writerbeware.org
Science Fiction Writers' Association site that gives lots of info about agents, as well as publishers and the like. You don't have to be a science fiction writer to appreciate the great info here -- it's a great service that SFWA provides, akin to Preditors & Editors (see above).