Which non-D&D supplemental product should everyone know about? Give details.
I have somewhat mixed feelings, personally, about Robin Laws as a game designer. Not that he designs bad games - quite the opposite, in fact. But what he wants out of the games he designs seems to conflict with what I like to see in games I enjoy playing.
On the other hand, I always get something out of his thoughts on game theory, whether it's adventure design, how to be a better player, or how to up your game as a GM. In fact, I think the first time I was really made aware of who Robin Laws was was when I picked up a short booklet put out on the latter topic, Robin's Laws of Good Game Mastering.
It's only available in PDF nowadays, but I still have the printed version I picked up soon after the book came out. At the time, I'd been running games for some years but I felt I could be doing a better job. Robin's Laws didn't exactly contain anything I didn't already understand, but it helped organize and clarify a lot of concepts and undoubtedly sharpened my GMing skills. I've largely internalized the contents of the book at this point, but I occasionally go back and flip through it nonetheless. It packs a lot of great advice into a little less than three-dozen pages, and is, in my opinion, required reading for all GMs.