The Zuse Z1 in its residential settinguse's third machine, the Z3, built in 1941, was probably the first operational, general-purpose, programmable (that is, software controlled) digital computer. Without knowledge of any calculating machine inventors since Leibniz (who lived in the 1600's), Zuse reinvented Babbage's concept of programming and decided on his own to employ binary representation for numbers (Babbage had advocated decimal). The Z3 was destroyed by an Allied bombing raid. The Z1 and Z2 met the same fate and the Z4 survived only because Zuse hauled it in a wagon up into the mountains. Zuse's accomplishments are all the more incredible given the context of the material and manpower shortages in Germany during World War II. Zuse couldn't even obtain paper tape so he had to make his own by punching holes in discarded movie film. Because these machines were unknown outside Germany, they did not influence the path of computing in America. But their architecture is identical to that still in use today: an arithmetic unit to do the calculations, a memory for storing numbers, a control system to supervise operations, and input and output devices to connect to the external world. Zuse also invented what might be the first high-level computer language, "Plankalkul", though it too was unknown outside Germany.
The title of forefather of today's all-electronic digital computers is usually awarded to ENIAC, which stood for Electronic Numerical Integrator and Calculator. ENIAC was built at the University of Pennsylvania between 1943 and 1945 by two professors, John Mauchly and the 24 year old J. Presper Eckert, who got funding from the war department after promising they could build a machine that would replace all the "computers", meaning the women who were employed calculating the firing tables for the army's artillery guns. The day that Mauchly and Eckert saw the first small piece of ENIAC work, the persons they ran to bring to their lab to show off their progress were some of these female computers (one of whom remarked, "I was astounded that it took all this equipment to multiply 5 by 1000").
ENIAC filled a 20 by 40 foot room, weighed 30 tons, and used more than 18,000 vacuum tubes. Like the Mark I, ENIAC employed paper card readers obtained from IBM (these were a regular product for IBM, as they were a long established part of business accounting machines, IBM's forte). When operating, the ENIAC was silent but you knew it was on as the 18,000 vacuum tubes each generated waste heat like a light bulb and all this heat (174,000 watts of heat) meant that the computer could only be operated in a specially designed room with its own heavy duty air conditioning system. Only the left half of ENIAC is visible in the first picture, the right half was basically a mirror image of what's visible.
Two views of ENIAC: the "Electronic Numerical Integrator and Calculator" (note that it wasn't even given the name of computer since "computers" were people) [U.S. Army photo]
To reprogram the ENIAC you had to rearrange the patch cords that you can observe on the left in the prior photo, and the settings of 3000 switches that you can observe on the right. To program a modern computer, you type out a program with statements like:
Circumference = 3.14 * diameter
To perform this computation on ENIAC you had to rearrange a large number of patch cords and then locate three particular knobs on that vast wall of knobs and set them to 3, 1, and 4.