Arduino is a development platform for rapid prototyping of simple applications. There are many Printed Circuit Boards (PCBs) that easily plug into the Arduino board. In my UF Small Satellite Design Club microgravity project, we needed a second Arduino board to increase the number of IO (Input/Output) ports but keep the X-Y dimensions smaller (which is why the Arduino Mega was not an option). This project is a realization of increasing the number of IO, Vcc, and GND pins using the on board I2C library.
Editor's note: I'm still adding content of all my projects. I will upload schematics and pictures ASAP.