This is one book that no children's writer (or aspiring children's writer) should be without. In addition to listing just about every children's publisher and magazine known to man, Children's Writer's and Illustrator's Market (CWIM) also includes great articles by both famous and newbie authors. So, check it out at your local bookstore or library. Oh, and while you're at it, check out Alice Pope's blog. She's the editor of CWIM and has some great stuff every week, like interviews of top authors and publishers, news about the writing world, and other fun "insider" tidbits.
The 2009 edition of CWIM is coming out sometime in August, so don't rush out and buy the 2008 version. Better wait and get the most recent version.