Nikon makes three extension tubes: the PK-11A (8mm), the PK-12 (14mm), and the PK-13 (27.5mm).  They used to make a tube called the M-ring, which is also 27.5 mm.

They used to make a tube called the PN-11, which is 52.5 mm.

All these tubes preserve the automatic diaphragm, but not the autofocus, although they can be used with autofocus lenses.