Arduino (Or Raspberry Pi) As HID Device
The Uno cannot be used as a HID device through the on board USB. You might be able to use the V-USB Library to be a HID device using the digital IO pins. See this tutorial.

There are several arduino compatible boards that do can act as a HID device natively. Most boards built around the ATMega32u4 can be a HID device.  For example the Arduino Micro

See this question about the Raspberry Pi.