Which poisonous snake can spread the skin at the back of its neck to make itself look bigger?

The Cobras of Africa and Asia spread the ribs at the top of the spine to for the characteristic 'hood' when threatened.

The primary reason is to intimidate it's foes and to make it's self look bigger. This action (often including a loud hiss) is usually enough to deter predators from attacking.

See related link for a picture.