Nice subject!

My wife carries:
An LED keychain light with an on/off switch.
Spare Cash in the cars (I never used to think it was a good idea... she convinced me and now we carry enough for a gas fill and a nice meal out). She generally carries a bit of spare cash in a pocket as well as her wallet in her purse.
I've stuffed a small FAK into the diaper bag.
Compact Mirror

I've bought her a Fisher Space pen, but she's still doesn't carry it.
I'm going to buy her a LM Micra or maybe even a Juice - I think I can convince her of the utility of scissors and nail file, etc..
Lastly I'm going to get her to carry at least a lighter. Neither of us smoke and she feels there's no need, but....

Plus she has all the standard mommy things and generally is never found outside the house without at least .5L of H2O and some snacks like trail mix or goldfish crackers.