I don't know of a list, but I could list a few places for you here (mayhap this thread shall become it!!!).
Bastion: Greek Gods (or is it Norse? One's free the other isn't...), Ink & Quill, critters, magic items, and various other goodies.
Mongoose: Several previews, web enhancements for print-products (check out the new race of Demons... Well, it was new 2 years ago...).
RPGNow: Granted, you have to *look* through the material, but there is a crap-load of free stuff there.
MonteCook: While not pdfs, he has a number of on-line articles that can be copy/pasted from. So do most other former (and current) WotC employees.
Mongoose: Some web enhancements, and check out Fighter's Corner (then write MEG and tell them to write more, dammit!).
AgainsttheShadow.org: While their netbook is specifically geared towards Midnight, it's got some quality stuff in it. Well done material by a dedicated group of fans.
Athas.org: Updated Dark Sun rules (if you like your Dark Sun to be just like D&D).
Ehr... That's all I can think of at the moment...
Except my own stuff that's going up during the next week or so (starting with a revamping of the Fading Suns psionic system).
) of freebie PDFs relating to d20 or RPGs in general, that are also downloadable or point to a location where said PDF can be downloaded?
