Pocket knife
That most men, even businessmen, carried a pocket knife used to be a given. A man didn't need to be a sportsman or a hunter to carry a knife in his pocket, because most people recognized the handiness of having a small blade available to do a variety of things. Today, it seems that fewer and fewer men (and women) carry a knife because knives are seen primarily as weapons of criminals instead of the indispensible and multi-use gadgets they really are.
Today, most people are much more likely to have a cell phone, a PDA and a beeper in their pocket than a pocket knife. And while you might not be able to board an airplane with one in your pocket, for everyday traveling to work or about town on your day's errands, having a pocket knife with you can make quick work of opening packages and mail, unscrewing a screw in the absence of a screwdriver, and using it for any number of little things that are much more easily done with a tool. While you probably won't use a pocket knife every day, there will be that day when you realize that if only you had a knife, you could whatever you needed to quickly and easily. And in the case of an emergency where something needs to be cut loose quickly, like a seatbelt, or something needs to be cut for use in an emergency (think something like a tourniquet or a pressure bandage), a pocket knife can even be a life saver.
The basic pocket knife is a single blade that folds into the handle. You'll want to choose a pocket knife with a blade that locks opens, especially if you're not used to using such a knife. Older pocket knives could be inadvertently closed by users who put pressure in the wrong direction, causing injury to the fingers that were now wrapped around the handle and in the way. So look at safety first and choosing a sturdy, locking variety.
The blade doesn't need to be long as if you're carrying a mini katana, and in fact a shorter blade is better because it makes the pocket knife easy to carry. Most pocket knives' blades will come in between 3 and 5 inches in length. Some pocket knives have two or three blades of varying lengths and thicknesses, each with a notch on the edge where you can snap it out for use or a latch to let you open the knife with one hand.
The Swiss Army Knife is the grandfather of pocket knives, containing not just a knife blade but a bevy of handy tools, all in about the same amount of space as a standard pocket knife, and even smaller than some. These have been the standard in handy pocket knife excellence for 125 years so if you're unsure about which pocket knife to get, you can chose a Swiss Army Knife and you'll get from a few to a dozen tools or more, including a standard pocket knife blade.