I generally carry a small folder (Benchmade AFCK) and a small flashlight. I don't carry either for self-defense; as people have stated above, (a) the chances of having to use a knife for self-defense in most areas are virtually nil and (b) guns are better weapons.
That said, let me offer the following holzwege: I've had to change flat tires in the past and quickly grew tired of the rinky dink little lug-bolt wrenches and short jack levers that they give you with your jack and spare tire. So for the past I've carried a hex-nut wrench with a pivoting head that fits into a four foot length of hollow steel pipe, and a one-ton jack in the boot of my car. Now everytime I have to change a flat tire at least I don't bark my knuckles on the curb or give myself a hernia - and the job gets done fairly quickly. Now I may never use that four-foot hollow pipe as a weapon during my lifetime (at least, I sincerely hope not); however, the pipe is "ready-to-hand" within the equipment-context of servicable weapons in the world (as my friend Martin would say) along with folders, flashlights, guns, and, well, cars...
My folder, that is to say, is equipment with which to cut string and salami - for the most part.
Best,
Steve