The most flammable substance in the world is chlorine trifluoride, according to About.com. This substance is so flammable that it does not require an ignition source to combust.
Chlorine trifluoride is a corrosive gas that is colorless and toxic. It's also available as a greenish-yellow liquid. It's capable of instigating combustion of virtually any material, including many that are generally considered to be fire-safe. Chlorine trifluoride is capable of burning flame retardants, asbestos, glass, concrete and sand. It's used in the production of uranium hexafluoride, UF6. Chlorine trifluoride is also used in semiconductor production, and it's an ingredient in rocket fuel and an industrial cleaner.