This cellphone patent from Motorola lets a cellphone keep an eye on its own battery power, and when low, turns the phone into a parasite on neighboring phones. When the juice on your phone gets down to a certain point, the phone switches off all non-essential functions (like dialing and texting) and finds a compatible phone within Bluetooth range to use as a proxy. Once a phone is found, that's where it gets interesting.
Your low-battery phone will then send a Bluetooth message to the nearby phone, asking for its permission to act as a proxy. If given, your phone will use that phone as a relay to send an SMS message to everyone on your emergency contact list that you can still communicate but only via SMS. The SMS messages will actually be sent not by your phone, but by the proxy phone, which will pass messages to and from your phone over Bluetooth. An interesting application, but one that we can't see catching on unless SMS plans get a whole lot cheaper. That and if people warm up to the idea of
Patent [USPTO via New Scientist via Textually]