There's a small set of things I always have in my pockets. My cellphone. A pen. An LED flashlight. These are all useful to me almost every day. But the thing I've carried most consistently over the past few years is a Spyderco Delica, aka the "Clip It"
The Delica is small, lockback folding knife. There are lots of these kinds of knives, but the Delica stands out in a few ways.
First, it has a clip on the side that allows me to hang it on the corner of my pocket; the clip is reversable so I can carry on either side of my body (and I do). This has a few benefits. First, it's just way more comfortable to carry this way; it's not turning sideways in the bottom of my pocket like the Swiss army knife I used to carry. Second, I always know exactly where it is and in what orientation.
This leads to the next benefit. I can open and close the knife with one hand, quickly if necessary. Invariably, when I need to cut something, I'm already holding it.
Next, the knife is super comfortable to hold and use. It's small and light (and short enough to legally carry virtually everywhere). The bump on the back of the blade is a perfect place to rest my thumb. Together with the slight curve on bottom, I feel secure that I can push a bit on the blade with less risk of sliding forward. This alone makes it preferable to me than most of the straight handled folding knives.
Finally, it's got a good blade. It's sharp and holds an edge. The blade comes in three configurations -- serrated, smooth, and combination (half serrated, half smooth). I prefer the smooth or combination blades. Spyderco also makes a trainer version with a dull blade.
While the knife looks purposeful and a bit scary to some (and I'm sure it would be useful in a fight), I find myself relying on it almost every day for some mundane task -- cutting down a box for recycling, peeling an orange, cutting out a newspaper clipping. I use the knives so often that I am acutely aware now when I don't have one (like on flights after 9/11.) These are useful tools, well-designed and well-made, and relatively affordable. What more could you ask for?