Waterproof folder for keeping all your important bits and pieces dry.
Lots of talk about using sandpaper to roughen the pages before writing on it in chinagraphs / using waterproof / permanent ink, keeping small bottles of alcohol to wipe it clean etc....
Come in lots of sizes A4 for armour A5 for dismounted Inf etc. Knicking Curry house menu folders was very in vogue in the 90's.
An interesting use of the Nyrex can be seen in Morale
A series of useful inserts can be here E-nirex