I have this NYKO Power Grip for a Nintendo 3DS
Clicks right onto the back of a normal 3DS (not the XL)
Adds ALOT of extra battery time to your 3DS
Can Charge the PowerGrip and the 3DS at the same time with the port at the top
Grooves in the back are perfect for your fingers to fit in, making it alot easier to hold your 3DS without it slipping around