I remember, long ago (like last year), when I started querying. I was sooo overwhelmed. I didn't know how to find an agent to query. I started stalking all my favorite authors and trying their agents, but it was a time consuming process.
And then I found Writers Market. Writers Market publishes a book every year that weighs as much as my youngest child. It has lists and lists of agents, publishers, contests, ect. But going through it takes forever, especially in the newer versions that don't separate it into genres.
And then I had to make a spreadsheet to keep track of the agents I did query. I like spreadsheets, so this wasn't a huge deal, but I kept thinking, there has GOT to be a better way.
I don't remember how, but I stumbled across querytracker.net. It made everything so easy that at first I was thinking, this has got to be some sort of scam.
What it is is a database that has nearly every agent out there. You can search by name, agency, or genre. It keeps track of everyone you have queried, when and how they responded. It has a comment section where others who have queried can post genre, when they queried, and when they got a response, and any other comments they might think helpful to others. And it generates a report on each agent showing their number of responses - like whether they requested fulls or partial. There is also a forum to chat with other members, and a resource for writers section. And this is all for free.
There is a membership, that gives it's premium members additional, more in-depth reports and other benefits. I haven't used it, but I've heard lots of people say it's totally worth the $35 a month.
My only regret with querytracker is that I didn't find it sooner!