Let's see, has Peter Ball been mentioned yet? Sure, he's a co-worker of mine, so perhaps this might count slightly as favoritism, but he really has a lot of great flavor ideas. I personally consider myself fairly good at adding good fluff whenever I write something, but Peter does simply a great job meshing intriguing setting and story ideas with whatever rules he's working with.
The first thing he ever wrote for Nat20 was an article in Asgard, about warrior schools, and the sample schools he presented inspired me to actually use some of them in my campaign. Then he wrote four sample locations for Tournaments, Fairs, & Taverns, and in my opinion they helped make the product great.
Now he's co-writing The Adventurer's Guide to Surviving Anything with me, for ENMag, and having help out is a great boon.
So yeah, Peter, I'm a fan. Write more.