Baen Books:Baen is the specialist science fiction imprint started by Jim Baen. Site has a free e-books archive of their authors' in-print works.
Bantam: SF imprint owned by the publisher Transworld.
Bantam Spectra: Science fiction imprint of Random House.
Carroll and Graf: US publisher.
Collins & Brown: UK publisher of the Paper Tiger imprint.
Daw Books: Science fiction imprint which is owned by Penguin Putnam.
Del Rey Books: SF imprint behind writers such as Eddings, Brooks, McCaffrey, Turtledove, Clarke, Hambly and Chalker.
Orbit: British imprint for science fiction books owned by Time Warner.
Penguin Group: US publishing group which owns SFF imprints such as Daw, Ace and Roc.
Telos: British publisher of SFF works, including many Doctor Who related tie-ins.
The Black Library: Book imprint of science fiction game publisher Games Workshop.
Tom Doherty Associates (TOR): Specialist SFF imprint based in the US.
Voyager Books: Voyager is the current SFF imprint of publisher HarperCollins.
Warner Aspect: SF imprint of US publisher Warner Books, Inc.
Wizards of the Coast: Publisher of various RPG novels based around Dungeons and Dragons and their other game worlds.