Querytracker.net - Great source for agent information, as well as an awesome tracker to keep track of how many agents you have contacted and their responses. This site also has a wonderful forum that you can become part of where like-minded writers can help you keep mentally sane while you are pursuing the agents. Also there is a yearly fee if you want to, which gives you access to further tools to make the search more organized and provide you with more information you've built up.
Agentquery.com - Another great reliable source of agent information. I use this site to cross reference the information between the agent's website (Which is the most important place to check) with Querytracker and here to know the most up to date information. If it varies at all, trust the agent's website first though. This site also has a section where you can set up critique groups for your writing, as well as other discussion groups using the AQ Connect feature which is free.
So these two site are ones that I frequent and think you should too.