RS232 to TTL Adaptor (MAX3232)
This is a tiny little board that converts an Arduino or chipKIT's TTL UART signals into a proper RS232 signal. This is needed for interfacing with some industrial equipment and sensors, or with a traditional serial port (e.g., 9-pin D socket) on a computer.
It is based around the MAX3232 chip, so operates from 3.3V as well as 5V. It's really small, so it could play the part of a "bump in the wire" as a signal converting cable.
3.3V or 5V
Maximum baud rate