As most people are by now aware, we are down to a "big five," along with a number of smaller presses. The latter, of course, get far less attention from booksellers, reviewers, and traditional marketing/promotion outlets (e.g., radio talk shows), but they remain part of the "traditional publishing" route, as opposed to the self-publishing options (digital and hard copy via Amazon, Ingram/LightningSource, Lulu, et al.).
The "big five" are the following: Penguin Random House (Penguin, Dutton, Putnam, Random House, Alfred A. Knopf, Doubleday, Crown, etc.), Simon & Schuster ( S&S itself, plus Poseidon, Pocket, et al.), HarperCollins (which includes also Harlequin and others), Macmillan (along with St. Martin's Press and more familiar houses), and Hachette (Little, Brown & Company and other imprints). Details about these are explained in this marvelous graphic: http://almossawi.com/big-five-publishers/.