I would have to say probably the most under utilize PC peripheral is the external hard drive. You'll be quite surprise how many people actually don't have one of these at their disposal for data backup. Well good news, external hard drives over the year have dropped immensely in price. The Seagate Expansion 2TB USB 3.0 External Hard Drive is a steal at $79.99 at Micro Center.
The external hard drive is compatible with both Mac OS X and Windows and uses a USB 3.0 interface which come with a USB cable. Performance wise this external drive is pretty good with decent data transfer speed. I have two of these sitting right on my desk,so I can attest for good performance.