Slayer's Guide to Hobgoblins was released in 2001 - I think it was the summer or late spring, but I can't easily find any info saying exactly when.It was huge right away
Edit: I was not claiming one product (and to be picky I listed TWO), but was demonstrating the environment.
companies have changed names and it gets hard to keep track. But Sword and Sorcery Studios, Green Ronin, Necromancer games, and others hit the ground running. (There was freeport stuff at that Gencon)
Mongoose and Goodman were right in there, Fantasy flight did some quick stuff. And the desk shop 1-dude publishing companies went nuts.
Goodman started with the stand-alone d20 RPG Broncosaurus Rex in 2001, and didn't really get into the sourcebook game until 2002 with the Complete Guide series, and the first DCC adventures were released in 2003.
Fantasy Flight Games were certainly early adopters, with a fair amount of product released in 2001. It's hard to tell if they were early or late 2001 though.
I certainly wasn't suggesting that it took a year or so before anything happened with the OGL/d20 system. I'm saying it took about a year for it to explode. To some degree, that's because it took some time to actually make the product after the rules were released.
Basically, the first year was a combination of people with an inside track (Green Ronin via Chris Pramas), enthusiasts who wanted to be RPG publishers with varying levels of quality (with Necromancer Games being on the top edge of that), and White Wolf and FFG dipping their toes. When people saw that things were working out for these, that's when the boom really started - but that took like a year from the release in 2000. And of course, RPGNow was a major boost as well, which lowered the bar to publishing by quite a bit.