Zuse Z1 (1938)
Built by Konrad Zuse in his parents' living room in Berlin, the Z1 was the first
freely programmable binary computer. It used thin metal sheets as mechanical logic
gates -- sliding plates that implemented AND, OR, and addition. The machine used
binary floating-point representation with a 22-bit word length.
The original Z1 was destroyed in a 1943 Allied bombing raid. Zuse rebuilt it from memory in 1989. It was unreliable due to manufacturing precision issues with the hand-cut metal plates, but the design was brilliant and ahead of its time.