Resource review: Query Tracker
QueryTracker.net is an incredible database and querying tool. They keep very up-to-date records of all known lit agents, whether or not they are accepting submissions currently, and what genres they represent. On top of that, once you are logged in, you can also track whom you have queried, which agents have responded, and how much time has passed since your last interaction.
There are also some great social functions. Users can add public notes about the interactions they've had with particular agents, and the Query Tracker forum is very active. In particular, the forum is a great place to find other querying authors to swap feedback with—on everything from your query letter, first five pages, synopsis, to complete manuscript. (Check out the "Critique Group Central" channel to look for critique partners.)
QueryTracker.net offers both free and paid memberships. The free option is very generous. Many people will have all their needs met without an upgrade to premium. For $25/year, a premium membership gets you added search functions, the ability to track multiple projects, and several other features.
Of all the querying tools out there, Query Tracker is at the top of the heap.