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 Level Shifters is your friend in such cases, giving you two duplex channels of level shifting and taking care of the high/negative voltage generation all in a low cost breakout board. We use the trusty MAX3232E, a classic chip that is part of the MAX232 lineage so you know it will work great for all your RS-232 needs, up to 250Kbps.