All of the Tomes of Horror would be fantastic sources of monsters for Paizo. Great design, 100% open.
Green Ronin's Book of Fiends would also be a great source of demons, devils, and daemons. Well, maybe not demons -- that section was written by some Mona guy no one has ever heard of. I wonder whatever happened to him?
Green Ronin's Advanced Bestiary is a great source of nasty templates.
The Expeditious Retreat series of Monster Geographica books is a nice, no-frills selection of 3.5 monsters culled from all sorts of d20 products.
Lion's Den Press's The Iconic Bestiary -- Classics of Fantasy contains fantastic equivalents of signature closed content monsters. If you think you want a mind flayer, think phrenic scourge instead.
I know publishers are often very anxious about re-using good material for fear of customers crying that "I already own XX! Why did they have to waste 2 pages reprinting information if I already own XX!" And at a certain point, one could re-use material too much.
But for me, I would rather see some moderate re-use of existing, high quality material than a "new" rehash of a concept that's already done perfectly well. Don't recreate the wheel, especially when 30 different products have already tried.
And for that matter, this extends to other OGC material than just monsters. If you're going to include a new spell, make sure that Malhavoc's Complete Book of Eldritch Might or Necromancer's Eldritch Sorcery don't already have a spell that fills the same niche. Don't give me a completely new spellcasting base class that could be simulated using the eldritch weaver from Green Ronin's Advanced Player's Manual.
I don't expect Paizo writers to research every d20 book ever published, but all of the books I've mentioned in this post have wide critical claim, a certain measure of popularity, and work within the core d20 fantasy system. They, and maybe a half-dozen more, should form any d20 writer's canon of OGC works.
I think many OGL publishers have really failed the customers by not building upon existing, quality material from other publishers. I really hope that Paizo is able to lead the way on this front.