I used to carry a small moleskine notebook with me (I still use one of the larger A5 versions for taking notes at work) until I switched to 3″x5″ jotter cards, as featured in the hipster PDA. I went as far as having Crane make personalized jotter cards in a square grid pattern (Levenger’s are quite nice as well).

Interestingly, 3″x5″ index cards are actually 75mm x 125mm cards, as Melvil Dewey, the inventor of the Dewey Decimal classification and of the library card catalog system, was an advocate for the metric system.