There are plenty of knives out there you could feasibly carry around with you all the time—Gerber's got a whole suite of them. The latest to join the crew is an especially tiny lil' guy, but it could save your life.
The the Gerber Daily Carry Hook Knife is a itty-bitty, reliable, discrete little sucker that fits right into a sheath you can keep on your keychain. When extracted, it can cut you out of some restrictive clothes or a seatbelt in an emergency. Admittedly, that's not all that likely to happen, so in the meantime it can handle things like envelopes or freezer bags, or wire-ties.