Snakes:

• Any venomous snake.
• Vipera berus, a.k.a. the common European adder, a venomous viper found in Europe and northern Asia.
• Agkistrodon contortrix mokasen, a.k.a. the northern copperhead, a venomous viper found in the eastern United States.
• Acanthophis sp., a.k.a. death adders, a genus of venomous elapids found in South East Asia and Australia.
• Heterodon sp., a.k.a. hog-nosed snakes, a genus of harmless colubrids found in the North America.

Other uses:

• Adder (Farthing Wood), a character from The Animals of Farthing Wood.
• Adder (electronics), a digital circuit designed to do addition.
• AA-12 Adder, a Russian air-to-air missile.
• Adder, a Commanding Officer in the Nintendo Wars series.
• HMS Adder - any of seven ships of the Royal Navy.
• USS Adder (SS-3), an early US submarine.
• adder technology Adder Technology - KVM & A/V Manufacturing company