The Parallax USB to RS-232 is a great little adapter that takes your mini-USB connector and converts it to a RS-232 interface!
Although most PCs come with USB ports, there will always be a need for the simple, robust, easy-to-use RS-232 interface on which so many devices communicate. The Parallax USB to Serial (RS-232) Adapter lets you jack in your USB mini-B cable on one side, and get a regular RS-232 DB9 connector on the other side!
You may need to download the drivers for the FTDI FT232RL converter chip that's used on this adapter. FTDI is a very common IC for this purpose, so most modern operating systems already support it.