The thread about languages made me think about this...
What kind of notes do keep at hand for quick reference? As in, not just having the book, but having it in a more accessible form.
As a GM, I try to have a sheet with all "passive" skill checks of each PCs, always assuming they're taking 10 all the time, unless they specifically say otherwise.
This includes Perception, Sense Motive, Knowledge skills (I just ask them to roll a die and add the modifier myself so avoid metagaming. e.g.: I say something like "roll Knowledge(Religion) they will it's an undead creature, even if their character don't beat the skill check DC.) and languages known (in alphabetical order) so that I know who understands what.
As a player, I try to take notes about seemingly important NPCs. Basically their names and small summary of their appearance and behavior. Maybe a note about where/when we met.
If a NPC has a name, chances are I'll have to remember it later!
What about you, ladies and gentlemen? What kind of quick notes do you have behind your GM shield? (Other than the ones already present in the GM Shield itself)
Quick reference for spells, (reduced) monster stat blocks, combat action/AoO cheat sheet.
Will save, Perception, and Sense Motive modifiers are the ones you will probably needing the most. Knowledge skills I usually don't need to know in advance as we almost never actually use the option to identify creatures with knowledge skills.
Ah, yes, I keep their saves as well...
Although I've never needed to roll a hidden Will or Reflex save, there were times where the players faced undetectable poisonous gases, so I had them roll Fort save in secret.