A Yaesu VX8GR handheld HAM radio transceiver is my spare. I carry it in my work bag every day, along with a quarter-wave antenna, spare battery and a AA-battery adapter. It'll run for days as a receiver and can transmit at 5 watts - plenty of power to hit a repeater within earshot of someone who can call for help on my behalf. Although I work in a dense city, I'm not under the illusion that cell-phone infrastructure will be available for long after a major disaster, particularly an earthquake.

HAM radios are also very useful when venturing into the wilderness. Many hikers monitor the 2m/1.44m calling frequencies periodically for distress calls. And at the right altitude your meager 5 watts can cover tremendous distances.