Welcome to the Chrons!
Have you tried searching for older threads? There may be some hidden gold in older posts. I know there are several publishers (Tor, for instance, and DAW) who maintain an open door policy for direct submissions, but I have no idea how much time/energy these publishers have for the slush pile — probably not much.
Alternately, there are so many smaller independent presses out there if you're willing to step away from the Big 5 imprints, and most of those will take unagented subs. Again, research (on Chrons and in general) is your friend. I haven't looked up novel subs in a while, so I'm out of the loop, but maybe someone will come along with some ideas.