The world's first stored-program electronic digital computer successfully executed its first program on 21st June 1948. That program was written by Tom Kilburn who, along with the late F.C. (Freddie) Williams designed and built the machine. It was called the "Small Scale Experimental Machine", but was soon nicknamed the "Baby". It is also sometimes (unhelpfully?) known as the "Mark 1 prototype".

The Manchester Small Scale Experimental Machine -- "The Baby"