There are an array of clamps available which are perfect for nipple torture:

Adjustable Bell Clamps: Probably the most gentle of clamps - these have tiny bells which tinkle as your submissive moves around (light).

Adjustable Clamps: Can be tensioned to suit a person's individual pain threshold (light-medium).

Peg Clamps: Pretty much your standard clothes peg (medium).

Teeth Clamps: Give a nasty sting (heavy).

Weighted Clamps: Lots of pressure with an added weight for extra impact (heavy).

Other forms of nipple torture may include:

Hot Wax & Ice
Electrical
Deep Heat
Whipping, Strapping & Slapping
Pinching, Stretching & Twisting
Biting & Needles/Piercing