This USB Type C cable adds a DB-9 (DE-9) serial port to your computer or laptop. Works with MacOS X, Linux, and Windows. Note that this provides (approximately) +-6V serial (RS232) not 5V serial (TTL), good for old devices like GPS, handhelds, programmers, etc with chunky DB9's on the side. Some cheaper RS232 cables 'cheat' and only give you 0-5V which won't work with some devices - this one has a true voltage doubler/inverter to make it more compatible.

These are tested and work great with bit-bang serial programmers and adapters - note that they are slow compared to hardware serial ports due to the high overhead of USB.

They're now FTDI-chip based for better performance and reliability. The drivers are rock solid and available for pretty much every operating system you will run across. 

If you need to use RS-232 with a computer that has only Type A ports, check out this cable here.