Not being a mathematician, I was unaware of this gentleman before, but he has been described as the most important British contributor to the discipline before Sir Isaac Newton. Born 400 years ago today, he is known for introducing the infinity symbol still in use today. As this source says:
In his Tract on Conic Sections (1655) Wallis described the curves that are obtained as cross sections by cutting a cone with a plane as properties of algebraic coordinates [...] Wallis developed methods in the style of Descartes analytical treatment and he was the first English mathematician to use these new techniques. This work is also famed for the first use of the symbol ∞ which was chosen by Wallis to represent a curve which one could traced out infinitely many times. He used the symbol again in the more influential work Arithmetica infinitorum which was published a few months later.