SpareOne, an AA battery powered cell phone. You can use it to make to make an emergency cell call, or if you pop your cell phone's SIM card into it, you can use it like a regular cell phone. The battery will last forever, well, ten hours of talk time. There are some drawbacks -- although it comes in a waterproof bag, the phone itself is not waterproof, and it is only a cell phone, not a satellite phone. If you're out of cell coverage, you're out of luck. And you should check if it will work on your cell network. One online reviewer noted that it is a 2G phone and one of our local cell networks no longer supports 2G, so the phone didn't work on it.
This won't beat a sat phone, SPOT, or EPIRB if you're far off the beaten path and not in range of a cell network in an emergency, but it might be a good additional piece of communications gear if you're considering a backup for your cell phone.