Using the V-USB Software Stack for an AVR microcontroller, you can build one fairly easily with the included PowerSwitch project (schematics and program included).
Alternatively, you can create your own with an Arduino by using your PC's serial connection and a custom application to send the appropriate signals. I believe you can also use the PowerSwitch code with an Arduino, as they use Atmel microcontrollers. However, note that building your own circuit with the included schematic in PowerSwitch will likely be cheaper than an Arduino.