Please tell me I'm not the only one to mention Mike Mearls - reading his insights into 3E (and 4E!) has made me change how I look at the game itself. And Iron Heroes, of course, is one of the influences on 4e, so there's always that. (EDIT: I was beaten to it!)
James Jacobs goes on my list, because of what he's done for DUNGEON and, of course, PATHFINDER. Both of those products really shape how I look at D&D these days, and that's always something.
Roger Moore goes on the list, simply because of all the work he did on DRAGON. Kim Mohan, too, for the same reason. DRAGON was one of those huge influences on me when I was a young gamer.
Which brings us to Zeb Cook. While most of us aren't huge fans of 2e (I was reminiscing today about it with some friends, and we all realized what a horrible system it actually is), he did do Isle of Dread, which was a major adventure for me (and the first published one I ran, if memory serves). Plus, Planescape is one of those books that still echoes throughout D&D, even if the setting has officially been dead for almost ten years.
While people have mentioned Eric Noah et all sort of tongue-in-cheek, they deserve to be on this list. An online D&D community *has* absolutely shaped the game, and I think if we took a look at things, ENWorld (and other RPG sites) has changed in some ways how the game is actually designed.
Timothy Brown and Troy Denning, along with BROM, for making Dark Sun. I'm a huge DS fan, and for about two years, Dark Sun was synonomous with D&D. I'm still in love with the setting, to the point that I'll put up with 2e rules to play in a Dark Sun campaign. DARK SUN also showed D&D designers that a successful campaign world didn't have to be about gnomes, magical forests, and archmages; it could contain brutal, mature elements. I think it helped open the door for settings like Planescape, Al-Qadim, and whatnot. Though I may have my Chronology wrong.
Monte Cook takes the list, along with Johnathon Tweet and Skip Williams. Really the whole 3rd edition design team.
Rich Baker, Matt Sernett, and Frank Brunner, for Book of Nine Swords - such a change hasn't been felt in D&D since... Skills and Powers? But I think Bo9S was probably a lot more gamer-friendly.
Finally, Jesse Decker. Not only was his work on DRAGON greatly appreciated, but I really, really, REALLY love the Spellthief class he made. It is, to me, the coolest thing ever made for D&D, and I am always disappointed at how little support they have received.
(I purchased Complete Scoundrel SOLELY for the two spellthief feats and minor, minor support. That's how much I love the class!)