They wouldn't be my first choice for a cutting tool, but in an emergency or in a non-permissive environment, having one inside your wallet might prove very helpful. There are many models out there, some more expensive than others. The TOOLOGIC is a variant that has a plastic case-boy with whistle, compass, etc. and a steel knife that sheathes into the card. It's very nice and can be purchased for around 20 dollars.
BUSHCRAFT WOOD DEVIL