My first instinct was that it's a Wii peripheral because of the colour (the Wii has Gamecube controller ports under a flap).

The Wii famously has native wireless capability and Gamecube wireless controllers use quite small dongles that fit snug to the connector port.

Superficially it looks a bit like the controller converters that were made to allow PS2 and SNES controllers to be used with a Wii but I doubt it as it doesn't seem to have and hatches for the other ports.

It also looks like a third party variant of the Wii Speak.

Do you know anyone with a Wii to try it out on?