I would have to go with the srd as best resource for me, I use it all the time in playing and running 3.5 games and have never bought the 3.5 core books.
ENWorld would be #2
pdf would be Book of Beasts, I've used a bunch of stuff from there, it includes a couple hundred pages of 3.5 monsters.
Crystal Keep's pdfs would be a close second. I allowed a warlock in my game based on the info from there even though I don't own Complete Arcane.
Umbragia has a nice race book, Planewalker.com and Athas.com have nice pdf resources.
I also highly enjoyed the Dice Freaks Hell supplements and have used stuff from there.
The first draft Nyambe pdf from before Atlas Games picked up the setting is pretty neat too.