Yesterday's Guardian has a whole-page obituary of Israel Gelfand, who died last month.
Israel Gelfand obituary | Science | The Guardian
Gelfand and his collaborator Mark Naimark, along with John von Neumann, were the founders of the theory of operator algebras (of which my username is an abbreviation). This has developed into one of the more active strands of mathematical research in the past half century, with important applications in noncommutative geometry, knot theory, quantum field theory, quantum statistical mechanics and the representation theory of topological groups.
I met Gelfand a few years ago, at the International Congress of Mathematics. His mind was as sharp as ever, although by then he was well into his nineties. He was eagerly listening to young mathematicians explaining their work to him, and giving them ideas and advice.