My custom Phantom KeystrokerI made this after trying to fool someone with a Phantom Keystroker. This was the original version from Thinkgeek that was a bare circuit board, not an external USB dongle. I had made a custom cable to connect it to the internal USB header on the motherboard and hid it inside the computer case. It was found fairly quickly despite not being physically located since the thing shows up in the device list.
Mine disconnects from the USB bus when it is not active. It is also much smaller and can be plugged into a USB port directly. It is based on the Atmega328p and V-USB.
Purple PCB was produced at what is now oshpark.com. Hand soldered surface mount components. A couple of jumper wires to fix some traces.
This is the firmware running on it. All it does is blink the LEDs on the main keyboard (assuming it has them). The file is compiled in the Arduino environment and then the hex file loaded via an ISP programmer.
You can now do something like this with either a Digispark or Adafruit Trinket with little effort.