Roving Networks RN-240 Bluetooth serial adapter
You just can't buy a laptop today with a serial port, but most enterprise network devices need you to perform the initial IP address setup via a serial console. While a slew of Bluetooth serial devices are out there, the RN-240 Firefly from Roving Networks has been the only one in my kit for nearly a decade. I can literally plug this in and work outside the telco closet or across the room. Since it even works with my Nexus 7, I have lots of choices for hacking that initial config.