Best known for their works of fiction, Random House is what the publishing industry calls a boutique publisher. This means they cover most areas within the publishing world. From fiction to non-fiction, Random House carries a wide variety of books from all cultures. Their individual publishing groups, also known as imprints, include Batam, Del Rey and ESPN Books to name a few. To submit work to Random House, it's recommended that a writer have a literary agent. At this time, Random House doesn't accept unsolicited manuscripts. Submissions are accepted by writers with representation from a literary agent. Similar to commercial publishers, a real literary agent or agency will never ask for a fee. They get paid when the author sells their work by sharing a cut of the author's advance and royalties.
The Penguin Group is broken up into a number of smaller imprints. The imprint an author writes for will depend on their chosen genre and what age group the novel is geared toward. A few of Penguin's imprints include Ace Books, Firebird, Puffin Books and The Viking Press. As whole, The Penguin Group covers most genres within the publishing community. At this time, Penguin does not accept submissions by unagented authors.
HarperCollins covers most genres, much like Random House and Penguin. In most cases, this publisher doesn't accept unsolicited manuscripts. However, for submissions to their romance imprint Avon, they accept works of fiction from authors without agent representation. Avon publishes romance novels. The genres they accept change depending on what the market demands. For example, as of March 2011 they're looking for paranormal, vampire, steampunk or fantasy romances. To know what genres they're looking for, keep an eye on their submission guidelines.
Much like the other large publishing houses, Simon & Schuster accepts submissions for most genres. This particular publishing house has imprints categorized under two age groups: adults and children. A few of their imprints include Aladdin, Simon Pulse and Gallery Books. Simon & Schuster only accepts manuscript submissions from authors who have agent representation.