USB HID emulator.

I thought I'd ask a bit more of a specific question, since my previous topic was kind of vague, in the end!

I want hardware that emulates an HID device, such that when I send commands to it with my PC, it forwards those commands to the target USB host, which thinks that it's receiving those commands from a legitimate HID device.

Very common topic. If you have an UNO, no extra hardware is needed. You should probably use LUFA on the 8u2, like in this guide here: http://hunt.net.nz/users/darran/weblog/13a32/Arduino_UNO_Keyboard_HID_Part_1.html (third result with lufa + arduino)