Here is my list.
That's easy, grab Philip J Reed's books. I am sure there is at LEAST one, that any GM could use.
Construct Mechanus
Constructus is more specialized, but it could make for one hell of a foil
101 Spell Components has some cool new rules, but again, pretty limited.
101 Mundane Treasures: More treasures. Very cool. Great for a GM.
101 Spellbooks Tomes of Knowlege and Forbidden Grimoirs. Probably my favorite.
In the Saddle, looks REALLY good. I have a friend that has it, and I like it. I plan on getting it soon.
Elements of Magic, also looks good, but it is more of a special use book, though, you could base your entire magic system, if you make a custom world, off it. I plan to use it for some NPCs, to make them quirky, and keep my players off-balance.
Book of Templates is pretty cool... I think you need to tweak some templates a TOUCH, or the CR... but they are very good.
If you have bards, Enchiridion of Mystic Music is very good. If you have a soft spot for martial artists... I would highly recoommend Beyond Monks.
If you like Psionics, get the recent stuff from Malhavoc, or Psionics Companion.
One that my friend really suggests, for the Paladin types out there, is Forgotten Heros: Paladins. He loves that book.
Swords of our Fathers looks really cool, though, it is pretty specialized. And, requires a bit more GM work, IMO, than the other pdfs above.
You also kind of have to mention all the Malhavoc products. The stuff is all above average, but you pay a premium for them, and then they come out a few months later, in print form. I personally think you are better off going with some of the above books.