German polymath: mathematician, philosopher, theologian, and scientist (sort of). His development of the differential and integral calculi is one of his greatest mathematical achievements. Newton also developed these calculi, but Leibnitz's notation is commonly considered superior to Newton's, which is why we write, for example, d/dx rather than x with a dot above it.
In philosophy, he made many important contributions. One of his major goals in philosophy was to provide a sound logical proof of the existence of God. To do this, he reintroduced the ontological argument, probably first formulated by Anselm and also developed by Descartes. In doing so, he introduced the notion of a possible world, which is still in use by analytic philosophers today. Additionally, he composed a monumental metaphysical work, The Monadology, which postulates a world composed of simple entities which he called monads. Each individual being is a monad, in this theory, even God. The world of monads is deterministic, everything having been set in pre-established harmony by God at the creation of the universe. (That God is a monad makes this concept quite puzzling but still interesting.)
In addition to his heavily theological philosophy, Leibnitz also looked for a way of creating a perfect logical language. This attempt, which was unsuccessful for him and just about everybody else who tried it, is known as Leibnitz's Dream.