Duotrope - a resource for tracking submissions and getting to know different markets (for a small fee), including average response times, acceptance rates, interviews with editors, etc.
The Submissions Grinder - a resource for tracking fiction and poetry submissions. Newer, less established, and fewer users than Duotrope, but completely open/no fee
The Review Review - a resource for getting to know lit mags
Entropy Mag's Where To Submit - quarterly lists of presses, chapbook publishers, and lit mags open to submissions
Who Pays Writers? - a site where writers report who, how much, and when they were paid by magazines/ sites
Perpetual Folly - writing tips and annual rankings of lit mags
Bookfox - writing tips and less-than-annual rankings of lit mags
Catapult - offers online writing courses, in addition to their online lit mag