A vest with a billion pockets is a very handy garment.
On some days, I empty my attache-case into my vest, and bask in the warm feeling of being totally equipped with
  1. notebook
  2. camera
  3. pens
  4. mints:
  5. handkerchief
  6. pocket knife and pen
  7. reading material
  8. Palm V and keyboard
  9. cigar, cigar cutter, lighter
and having free hands at the same time.

When buying a vest, get one that also has a large, side opened, pocket in the back. Hunters use it for quarry, but you may find it useful for a sweater, or for some other item you don't mind sitting upon.
At security checks, it is convenient to just dump the vest and all the metal gear it contains into the X-ray machine, and be done with the stupid beeping.

Paired with cargo pants (the kind that has large thigh pockets), a good vest can turn you into a human cargo. It may also get funny looks.