A couple of weeks ago, I mentioned my $200 knife. A lot of frugal people might think that's an exorbitant purchase, considering that pocketknives may be had for $3.95 and up on Google. I'll wager very, very few knives that cheap have blades made of steel as good as the S30V in my Benchmade 943SBK.
The point is that a knife is, beyond doubt, essential gear. The convenience of having a knife close to hand makes everyday life quicker in hundreds of tiny ways. Aside from the everyday - opening letters and packages, crafts projects, etc. - a simple bladed tool increases survival possibilities in all kinds of situations. Art of Manliness has a post on the subject, and while it's largely tongue-in-cheek about the uses of a knife, it does give a good overview about the basic types of knives and agree with me that every man should carry one.
Your knife requirements may vary depending on what you do. I want a knife that is as light and thin as possible, so that it doesn't create bulk in my pockets and weigh down my pants. I want a knife that has good hard steel that will take and hold an edge. I'm not interested in multi-tools for daily carry and have no need of a quill knife or fishing knife, so a single blade suits me just fine. I do need one blade to be a clean edge and one partially serrated for cutting rope or twine, but a “combo” type blade can cover both those needs at once. For ease of carry, a pocket clip is a great addition as well.
Some people advocate switchblades; they're also called automatics or assisted openers. The legality of those types of folding knives is a nightmare, and I can't think of any reason to build extra complexity (read: more stuff that can break) into the simplest and most elegant tool I own. Lastly, I need an ambidextrous, one-hand open and close lock. I've been cut too many times when friction-hold knives or inferior lock designs collapsed under the slightest pressure; I no longer trust knives without good locks. The AXIS mechanism on my knife locks the blade into both the closed and open positions, so it's great.
I own several knives. One cost me $5 and has a plastic handle, a crappy liner lock, and a stamped, low quality blade. One was a gift from my father; it's a Case penknife with three blades, two at one end and one at the other - a great knife, but pretty thick around, no pocket clip, and no lock. A couple are multi-tools; great for camping but too heavy for daily carry. So I carry my jackknife and it hasn't let me down yet. If you don't carry a knife, I'd say you should consider it. It doesn't have to be fancy, it doesn't have to be expensive; it's just too useful a thing to go without.