All of these are the umbrella organizations for multiple companies and imprints. They change constantly as the industry shifts with the revolution in publishing. Most of their imprints require agented submissions. Smaller, independent publishing may not. Check web site submission guidelines to be sure. The complete list of companies and imprints is on pages 69-74 in 2013Writer’s Market.
Hachette Book Group USA – www.hachettebookgroup.com
Harlequin Enterprises – www.harlequin.com
Harpercollins – www.harpercollins.com
MacMillan US (Holtzbrinck) – http://us.macmillan.com
Penguin Group (USA), Inc. – www.penguingroup.com
Random House, Inc. (Bertelsmann) – www.randomhouse.com
Simon & Schuster – www.simonandschuster.com