Very helpful adapter which lets you plug in a midi keyboard, guitar or drums (selectable via a switch on the box) into Rock Band 3 and play it like a regular RB instrument. (Did not buy from here however as at the time of writing it was cheaper to buy direct from Madcatz). If you already own your own Keyboard with a MIDI out (please ensure you have MIDI out before you buy) then this is a great way to use it in RB3 without having to buy the keytar instrument.
Please note this item does not come with a midi cable out (I foolishly assumed there would be a fixed/built-in cable (like the USB) to connect to the midi port on the keyboard (or a cable included) - there isn't. So you will need to order a midi cable in addition to this adapter to use your midi instrument with it.