To be blunt, I hate that this item exists:
http://www.pfsdb.com/magicitems/hex-nail
What, the better spell list wasn't enough? You've got to make the witch even more inferior to the wizard by giving any modestly-prepared adventuring team and/or bad guy the ability to just roll into town and buy a dime bag of save boosters?
So, I'm trying to homebrew similar items that counter other casters. I'm thinking of grouping them into categories.
Wizard/Sorcerer/Magi (and assorted subclasses) could be countered by a Spell Blocks, little cubes of odd-colored lead of unusually light weight that contain tiny pieces of the null-magic that created the Mana Wastes. They are produced in Alkenstar, as one of its many unique exports, and work on dragons and other magical beasts as well as arcane casters. The lead blocks lose their odd color and become slightly heavier when they are consumed.
Clerics/Oracles and other "religious" divine spellcasters can be countered with Lawstones, little tablets carved with excerpts of the Laws of Man. They were invented, naturally enough, in Rahadoum, where many are actually mass-produced using concrete-pressing technology, and carried by government agents while abroad, to protect them from the influence of so-called "divine" forces. The secret of their manufacture has spread, however, and they can be found in many far-flung areas of the continent. They work against the abilities of Good and Evil outsiders as well as "religious" divine casters. Each tablet crumbles, with an audible metallic ringing sound, when it is consumed.
And, finally, druid/rangers and other "natural" divine spellcasters are countered with Firehobs, the remains of trees that have been rendered into charcoal while still living, then pierced with nails. These ancient, traditional counters to the influence of nature-worshippers are common across Golarion in any number of societies that have had conflicts with practitioners of the Green Faith, though not all are properly manufactured or made in the correct manner. They work on fey and plant creatures. When consumed, the nails pop out of the charcoal, which then crumbles into black dust.