Since I was a boy (in Scouts), I have always carried a pocket knife of some sort. I can't even begin to count the number of times I have used a pocket knife. It is right there with my wallet, watch & wedding ring.
When I was younger I carried a Wenger Swiss Army knife. This logged plenty of miles on camping trips!
My first job after school was in the rental-purchase business. I found that I always needed pliers, and both a standard & phillips screw driver, along with a knife. That was when I picked up my Needlenose Multi-Plier 600 Tool made by Gerber. I continue to carry this when ever I give someone a hand with a project.
Now that I work in an office, my every day pocket knife is the Leatherman Micra. Small, but with enough extras to solve most things I come up against (like tightening screws on glasses or fixing a jammed stapler )