If you ask most publishers - especially of the bigger houses - whether they ever accept unsolicited manuscripts, or the so-called 'slush-pile,' most of them will tell you point blank NO. For example, at the AWP Conference held this year in Manhattan, speaking on a panel about how editors acquire books, three out of the five representative editors said that most of their acquisitions, if not all, are agented (Norton, Harper Collins, and Viking).
But it is fair to ask, is acquiring an agent the only possible way for a writer to succeed at getting published and gaining attention? Apparently not, since there are publishing houses who have voiced a different opinion and who are willing to make direct contact with an author as a feasible option.
At the same AWP editorial panel, Johnny Temple, editor of Akashic Books, an independent publisher, accepts unsolicited manuscripts from unagented authors of mostly fiction, noir and some political nonfiction. He also accepts emailed cover letters and bios. His philosophy is, 'how about going directly to the writer?' He enjoys the idea of getting to know the author well. Further, he says that requiring that authors have agents is a very narrow view; he doesn't like the idea of big houses competing over the same book. What about the other great books out there waiting to be read? His small press moves off the beaten path.
But how does his staff of four or five read through all the manuscripts they receive? They just do. While they are at present on hiatus in accepting new manuscripts, they are paying close attention to promoting the manuscripts they have already accepted. Kate Nitze of MacAdam Cage has also found a way to alleviate the problem of unlimited unsolicited manuscripts; her house accepts unagented manuscripts for a limited time - Feb. 1 - June 1 of each year.
So if you are one of those writers who likes to move off the beaten path, consider the alternative.
One final note: Do your homework and research the internet and other available resources such as Writer's Market for those publishing houses willing to take on unagented authors like yourself.