If you're looking to interface with telco, retro or industrial equipment you'll probably run into RS-232 interfaces. The Adafruit RS232 Pal - Two Channel UART to RS-232...
$3.95
In Stock
Make your own custom DE-9 cable or DE-9 interface with ease, using this DE-9 Socket to Terminal Spring Block Adapter. You get a nice solid PCB with a DE-9 socket...
$6.25
In Stock
This USB cable adds a DB9 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...
$17.50
In Stock