I've been using Duotrope for a few years now. For those of you who have never heard of it, Duotrope is a site that lists over 3600 markets for fiction and poetry. It's free to sign up. The submission tracker allows you to enter the name, genre, and word count of your manuscript and helps you stay on top of your submissions by tracking days in the slushpile, your rejections, and acceptances.
I really like their monthly newsletter which lists publishers, their calls for submissions-including the themes, and the deadlines.
BUT...I recently learned of a fairly new website that also connects writers to publishers. At Hey Publisher, you can sign up for a free writer account and set up a profile. You can upload your work, browse publishers, and submit. Hey Publisher uses Amazon-provided servers and have a 100% guarantee your work will never be lost.
Sounds interesting and since I'll be putting my nose to the grindstone come January, I'll be signing up for Hey Publisher the first day of the new year.