This is one of those things that you would never think about needing...but once you get used to it, you can't leave home without it!
In Japanese, it's called a "card case", and it's kind of like those sheet protectors you put in a binder, but way nicer and way stronger. Sleeves like this are very popular in Japan, and they are in fact a staple of every Japanese office. OLs (office ladies, which is a generic term) shove sheets of paper in them to be moved from desk to desk and office to office.
So, many companies make similar items in every color and pattern imaginable, but this one here was selected because it's the best we could find. They are crystal clear, strong, and thin without being too thin nor too heavy. They are closed on three sides, leaving a single opening, whereas the more common variations are closed on only two sides. Why do we perfer a single opening? Security. There is less chance of stuff falling out, and it's easy to just shove these in a bag or brief case with the opening facing up.
We use them for all kinds of things, like for the safe storage and transport of pads, envelopes and half-finished letters. But our favorite use is for carrying travel documents such as boarding passes, passports, customs forms and itineraries - it makes getting through airports a breeze, with no fumbling around. You can even hand the entire sleeve with your boarding pass inside to the gate agent, and he/she can swipe it without taking it out!
Comes three to a pack. Comes in sizes A5, B5 and A4 - note that the measurements reflect the size of the paper that will fit inside - they are NOT the size of the card cases themselves.