Gottfried Leibniz is responsible for lots of stuff:
differential calculus and integral calculus, especially the basic notation that we use today;
the product rule of calculus may be called the Leibniz rule or Leibniz law;
differentiation under the integral sign? follows the Leibniz integral rule;
identity of indiscernibles? may be called the Leibniz law;
a version of infinitesimals sometimes regarded as a precursor to synthetic differential geometry;
a philosophy involving ‘monads’, simple mind-like substances, from which we get the names of both the monads of category theory and the infinitesimal neighbourhoods of nonstandard analysis, see at monad in nonstandard analysis
also see in Hegel, Science of Logic the section The monad of Leibniz