Perhaps the single most effective secret weapon ever, as some describe it. The composition of Greek fire was a highly guarded state secret, one that was lost forever when the Ottoman Turks finally captured Constantinople in 1453. Modern historians speculate it contained some sort of petroleum, or possibly phosphorus.
It burned on water, could be shot in any direction and was packaged into grenades and cannonballs, helping the Byzantines fend off Arabs, Vikings, Crusaders, various Turks and all manner of Italians. It could also be hurled by catapults over city walls or dumped on attackers trying to climb them.