Natural 20 Press accepts submissions, at least for writing; we usually contact artists ourselves, but we are always glad to look at someone's portfolio. I'm the point of contact for most submissions- RangerWickett@hotmail.com
Darkfuries does. I agree with Matt of Mongoose Publishing: many of the d20 publisher sites have a submission guidelines page, or at least a point of contact.
We also will accept submissions- both fantasy and The End (d20). Of course, we cannot guarantee we will print them, but we promise fast turnaround on that decision.
I think the best way to get a company's attention that you have something special is to self-publish on rpgnow. Initially, this will put (a little) cash in your pocket, let you know what the gaming community thinks of your idea, and allow you to get feedback and polish it.
Then, when you start sending emails to those contacts listed on the various company pages, you can say, "This is my idea, go to rpgnow and see what others are saying about it. If you're interested, you can see the finished product itself."
That's what I did three months ago, as a never before published writer. By the end of this year I will have 2 products printed. One of those being my very own "pet project" that I self-published, the other being something totally unrelated. Even before MEG decided to publish Vigilance, Doug liked what he saw in my work enough to offer me a spot on the Raw Recruits design team.
There is simply no better way to demonstrate your talent and ability than a 25 thousand word writing sample.