Aleksandr Friedmann: A biography with reference to contemporaries, and a list of references.
Georg Riemann: Although he died before the development of general relativity, his work in non-Euclidean geometries is very important to studying a curved spacetime.
Georgio Ricci-Curbastro: The mathematician who developed much of tensor calculus before Einstein found a use for it.
Hendrik Lorentz: Brief biography.
Hermann Minkowski: A brief biography with quotations and a bibliography.
Hermann Weyl: One of the first people to combine general relativity with the laws of electromagnetism
Karl Schwarzschild: A brief biography with reference to contemporary mathematicians and theories.
Karl Schwarzschild: The man who solved Einstein's equations and predicted the existence of black holes
Luigi Bianchi: A mathematician who developed many theorems regarding Riemannian geometry
Marcel Grossmann: A brief biography on Marcel Grossmann, a classmate of Einstein who was the first to find a connection between tensor calculus and the theory of relativity.
Southampton GR Explorer: An introduction to Einstein's theory of General Relativity and related topics. Includes text, pictures and movies.
Willem DeSitter: Mathematician who studied solution to general relativity and developed DeSitter space