This is a description of my first useful Arduino project - The Finger.
The external USB drive that's a backup device to my Mac Mini media server doesn't cope well with being on 24/7 ...
Checkout phatIO an IO device that looks like a USB filesystem
Set a pin to 5V by saving "1" to its control file, set it back to 0V by saving "0".
Control LCD and LED displays by writing the data to display to a file.
Communicate with TWI, and SPI and other devices by writing data to a file.
Videos, reference and more at phatIO