Most people think of nuclear bombs when they think of Weapons of Mass Destruction, but it is far easier and more cost-effective to use chemical weapons for similar and sometimes greater effect. The classic chemical WMD is Sarin Gas, a nerve toxin that has been used in various attacks around the world. There are treatments for Sarin exposure if caught immediately, but it acts so fast that most cases have died before medical help could arrive.
Several common chemicals have been used as chemical weapons. Chlorine is toxic in its purified form, and was used in World War I as a weapon. Both Phosgene and Hydrogen Cyanide are used in manufacturing, and were deployed during World War II.
Chemical weapons have a strong negative public bias, and are often seen as cowardly and needlessly cruel compared to faster bombs and bullets.