Recent Arduino boards including the Uno and our Freetronics Eleven board use the small ATmega8U2 microcontroller to take care of the USB interface. However this can also be programmed to help the board behave as a USB keyboard, and the people at Mitchtech have explained the process for doing this in a well-documented manner. Several examples are shown, including making a keyboard-based volume control, for example:
Certainly useful - you could make a macro-style function to enter many presses, perhaps a sequence to enter a password, or hide it at the back of the machine and enter random keystrokes as a practical joke. All this and more is possible with ourFreetronics Eleven - the Arduino-compatible with onboard prototyping area:
To find out more and learn how it's done, visit Mitch's website here.