Oh, well then, I'll have to "me too!" that MMS book, as well as From Stone to Steel. Both excellent OGL/d20 books, that cover their respective areas admirably well. For d20 books...
Mutants & Masterminds stands out as by far the best d20
system ever made, though I'm sure some random Spycraft aficionado will leap out of the woodwork any moment now and go all sniper on me.

It's so good that it's worth at least looking at to mine for system modifications, even if you'd never use it. Plus, you
can use it for things other than supers anyway - people do. Extremely flexible - as all supers games must be, I suppose - and full of tasty crunch, yet front-loaded, so once chargen's done, it's a smooth ride.
Toolbox is worth a mention, as being just incerdibly useful for a 3e DM. I don't believe the (IMO) even better (and/but system-neutral)
Ulimate Toolbox is actually OGL, let alone d20, so never mind that. Tables upon tables... I love that stuff. Ever since the AD&D 1e DMG, I have.
Um. Mostly I like some of the "small press" PDFs, for how innovative or bold they can be. Literally, *can* be - as in, "actual companies" mightn't be able to afford such risks... well that, and they'd never think to anyway.
I feel enthusiastic about a scarily tiny number of OGL/d20
books.
