30 Which non-D&D supplemental product should everyone know about? Give details.
I've never been much of a supplement guy. As a relatively well-read and imaginative kid (and a poor one to boot), we just made up everything for our D&D and Star Frontiers games. We didn't have many modules. We had few supplements. So when I grew up and started making enough money to buy other games, I never went overboard with buying supplements (with the exception of the original run of 3.0 splat books – Sword & Fist, Song & Silence, Tome & Blood, etc. and yes, I feel like I got kinda burned by them).
So I don't know of any supplements that I consider a must have. I've used supplements for inspiration, but it's usually just cherry picking what I like and leaving the rest.
So, no recommendation from me here, I'm afraid.
But if I ever put out a supplement for Flying Swordsmen or Chanbara, let me just say that it will rock your world on toast or whatever the kids say these days and you should definitely pay me large amounts of money for said future supplement, if and when it ever does appear. ;)
I promise, though, even if that mythical supplement appears, it will NOT lead to a vast churning pile of splat. Not that I don't have the ambition, I just really don't have the time!