Godfather of the idea that everything is made of atoms and empty space. The guy who turned Monism into a taboo doctrine.
It is said that he came up the idea of atoms by searching for a solution to the Arrow Paradox. He recognized that the paradox is unsolvable because the half-distance can be divided infinitely. If there was a particle small enough to be non-divisible, the paradox is solved. He believed that there were different types of atoms, and that "the type of atom determines the taste of thing".
He made the division of knowledge into true-born knowledge and bastard knowledge. Bastard knowledge being that gained through senses, while true-born knowledge is gained through reason (Senses are deceptive).
He is one of the first philosophers to dismiss the idea of afterlife.
He wrote more books in his lifetime than all other Pre-Socratic Philosophers combined.
His ideas were largely dismissed until 17th century. @Plato was against them to the point that he said that he'd prefer to see all his books burned. Finally, @Epicurus saw the potential of his teachings, but that didn't lead to their wider recognition, as Christian thought started dominating the world at the time.