So you may be wondering "heck, I should just grab an Arduino!" But a 'proper' Arduino can't do what we want, which is to appear as a keyboard. When you plug in an Arduino into your USB port, it shows up as a Serial device, which is fantastic for debugging or for interfacing to Processing. To listen to a Serial device, you need to open up Hyperterm or Zterm or the Arduino IDE's serial monitor. However, it does not act as actual keyboard where what it outputs goes to Microsoft Word or a video game.

The Arduino is a USB serial port - it appears under Ports here, not under Keyboards!

For that, we need a different kind of chip, a chip that is USB native! USB native chips can act as USB serial ports, but they can also act as MIDI devices, keyboards, mice, audio devices, joysticks, etc. Nearly anything! A nice chip that does all this is the ATmega32U4 (the U is for usb!) .

The Teensy 2.0 is basically this chip, a USB connector, button and some other necessary things. It's very tiny (thus the name) and has a fantastic programming interface that is basically the Arduino + a helper, it runs under Mac, Linux or Windows.

Since this tutorial was written, a number of other 32U4 microcontroller boards have been developed including the large Arduino Leonardo and the smaller Adafruit ItsyBitsy 32u4 - 5V 16MHz, Adafruit ItsyBitsy 32u4 - 3V 8MHz, and Adafruit Feather 32u4 Basic Proto, which can do similar things.