Bonus: Knock iPhone 'key' for Macs
OK, this isn't hardware, and it makes your iPhone a peripheral to your Mac, rather than provide an add-on to your smartphone. But it's too cool to leave out.
If you have a very recent Mac running OS X Mavericks and an iPhone 4S or later model, the Knock app lets your iPhone unlock your Mac when you come in range. It essentially serves as an automatic password for your Mac, similar to the Bluetooth key fobs that some cars have. In this case, your iPhone is your key fob, making it a Bluetooth peripheral to your Mac.