One of my fellow writer friends told me about a great site if you write short fiction. It also has info on book publishers, but that's easier to find anyway. I find that Duotrope Digest is particularly useful for folks who write short fiction because the market is so elusive--generally slowly shrinking but every now and then something new opens up. This site lists all kinds of genre markets and indicates whether they pay or not--and what the response time is, too.
It even has a submission tracker where you can enter your stories and how long it took to hear from the editor. It doesn't want you to put anything in until you've heard back, though. I wish you could enter the submission info as you send it, and the update it once you get the answer.
Still, a useful site. Check it out!