[GTALUG] Emulating a USB keyboard using ???

Hi guys, Do you know a way to emulate a USB keyboard using Raspberry Pi, BeagleBone Black, ODROID, or some other embedded board? That is, as far as PC is concerned, it's receiving keypresses from some keyboard. I know those boards have UART pins. But, even if I connect the pins to a serial-to-USB cable, the USB end of the cable will show up as serial (/dev/ttyUSBx) and not as keyboard. I have such a cable, and that's what I see. -- William