Hazardous Gas Detector: Just in case you didn't bring your canary to tell you if there's poison in the air or not, bring along a detector. They are no bigger than the palm of your hand and will save your life.
Chain Cleats: Since you might need to be navigating some treacherous terrain in your quest for safety, bring along these to give you some serious traction.
Personal Distress Light: Again, to help your rescuers spot you. There are several different types of these lights, but definitely bring one along with you.