Keychain Utility Knife?

Often at Home Hardware, by the cash.
 
My favourite is the Victorinox Swiss Army Knife, "Signature" model @ $37.50 Canadian.

Very small with a small metal ring to attach it to my larger key ring.

It includes a sharp knife, small but effective scissors, tweezers, an extendable ball point pen tip, and a metal nail file/slot head screw driver.

I find I use it at least once a day for various purposes.