I always carry my Swiss army knife or Leatherman. Swiss when I'm dressed well or going to be in town, Leatherman for country or outdoor work. and there is a small blade on my keychain at all times (it even makes it into courthouses etc. as it isn't recognisable as a knife)

I put More than one because Of the above and because if I am going to be in a "gun free zone" I carry a tactical folder as well unles there are metal detectors baring the way.