Moonratty has an excellent post about why authors should never submit unagented manuscripts to a publisher.
I agree with about 95% of the post. I would argue, however, that in certain kinds of genre publishing and with smaller presses, unagented submissions are expected and - in some cases - encouraged. Some larger genre publishers and many small indie presses actively solicit slush for any number of reasons. They wouldn't solicit slush if they didn't occasionally find a gem. But a manuscript referral from a trusted agent will still go to the top of the pile at these houses.
Even if an editor at one of these houses makes an offer on your unagented submission, do yourself a favor and try to get an agent before the deal is finalized. It'll be better for you in the long run, for all of the reasons outlined in Moonratty's post.