A mathematician who was an early pioneer of ComputerScience, who developed the concept of an abstract machine, with a RAM separated from the CPU by a Bus, and much work on automata (see FiniteStateMachine) and RandomNumberGenerators. (Of course, he didn't do all this by himself - some of his work built on AlanTuring's ideas.) Another area he is well-known for is GameTheory?.
Modern computer designs are sometimes known as the "von Neumann architecture", or perhaps "the von Neumann bottleneck" when talking about all data travelling back and forth along the Bus between the CPU and the RAM and HardDisk(s).
He was a Hungarian who left Europe for the United States as he saw World War 2 become inevitable. During the war he did calculations to determine the optimal altitude to detonate an atomic bomb to cause the most damage.