As part of a larger project, technologist 'Coronax' has demonstrated a method of allowing an Arduino to communicate with a Commodore 64 computer using the SPI bus. The Arduino needs to be set to slave SPI mode however this is easily accomplished. This allows the Arduino to enter text as a keyboard for the C64, and also allow a PC via the serial line to also act as a terminal for the classic computer. If you have the hardware and the time, the source code for the C64 and Arduino has been provided to try yourself.
If you're new to Arduino, the first step is a solid board for your projects - our Freetronics Eleven - the Arduino-Uno compatible with low-profile USB socket, onboard prototyping space and easy to view LEDs: