Well, this is what I have:

Pens, lots of pens. Pencils.
Notepad, larger pad of paper.
Yellow post-it notes, big & little.
White-out. Highlighter pens. Spirit markers.
Painkillers & plasters (note, giving these to people is a very
  bad idea under some legislatures, including US & UK. I allow
  people to steal them, but would *never* give them to somebody).
Safety pins. Needle and thread. (For when wardrobe have vanished)
Sellotape, LX tape, Gaffer tape - black and white.
Masking tape for marking up cue-lights.
Paper glue, stapler (good for costumes as well as paperwork...)
Screwdriver. Stanley knife.
Chocolate, for those endless techs.
Stuff to keep actors amused & quiet - this started when I did
 kids shows, but is handy for adult actors too - cards, travel games.
Wet wipes. Tissues.
Copies of company contact list, props list, local list of 'phone
numbers (printers, fire marshall, local hospital, places to get
*anything* at short notice).
Maglight or other torch. Dark gel for dimming down working lights.
Glow tape.

Probably overkill - if you have co-operative wardrobe & tech-crew
around they'll deal with the problems and you'll never need half of
this stuff.