If you want to interface with telco, retro, or industrial equipment, you'll probably run into RS-232 interfaces. The Adafruit RS232 Full-Pinout Breakout with 8 Channels of UART to RS-232 Level Shifters is your friend in such cases. It gives you 5 input and 3...

Make your own custom DE-9 cable or DE-9 interface with ease, using this DE-9 Plug to Terminal Spring Block Adapter. You get a nice solid PCB with a DE-9 plug and one line of push-spring terminal blocks that you can use with stranded or solid core wire to quickly assemble...

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 (TTL), good for old devices like GPS, handhelds, programmers, etc with chunky DB9's on the side. Some cheaper...