Harper Voyager, the scifi/fantasy imprint of HarperCollins, is accepting unsolicited manuscripts from October 1 to October 14, 2012.
This publisher does not ordinarily accept unsolicited submissions, so this is a big opportunity for speculative fiction writers with a completed manuscript. Please note that they are not accepting uncompleted manuscripts. Your novel must polished and ready to submit by October 14, 2012. Books that are accepted will be published digitally, not in print format.
You can find the guidelines here. Read them carefully.
You can submit your manuscript here.
FYI, along with your manuscript, you also must submit a brief synopsis, no more than 1,500 characters, including spaces, the best scene or first 1,000 words of your novel, and your query letter.