The Analytical and Difference Engines (1835-1869): The English mathematician Charles Babbage (1792-1871) never got to build his invention, but his design had an uncanny resemblance to the modern computer. Ada Lovelace, Lord Byron's daughter, wrote eloquently about the device and was history's first programmer.
The ABC (Atanasoff Berry Computer) (1938): John Atanasoff and Clifford Berry designed the first electronic digital computer at Iowa State, and urged the university to patent their design. Nothing was ever done and millions in potential royalties were lost.

The project was abandoned in 1942 without building the computer.
The ENIAC (Electronic Numerical Integrator and Calculator) (1946): was built at the University of Pennsylvania by John W. Mauchly and J. Presper Eckert. Recognized as the first operational electronic computer, it could perform 5,000 additions per second, weighed 30 tons, and required 1,500 square feet of floor space.
UNIVAC I (Universal Automatic Computer) (1951): The world's first commercial computer (a total of 15 were sold) gained public recognition when it was used by CBS to predict Eisenhower's 1952 presidential election.

The original UNIVAC was officially retired on October 3, 1963, after 73,000 hours of operation, and is now on display at the Smithsonian Institution.
The IBM 360 (1964): The 360 series pioneered the concept of upward compatibility whereby a user could upgrade from one computer to the next without having to reprogram existing applications. The machine was viewed as an enormous financial gamble, but paid off handsomely as it gave IBM a dominance in mainframes which it has never relinquished.
Altair 8800 (1975): The January 1975 issue of Popular Electronics featured the Altair 8800 on its cover: the first personal computer and a machine that the hobbyist could build from a kit. 2000 adventurous readers sent in their orders (sight unseen) for a kit that cost $439.

The Altair had no keyboard or monitor and no available software and was programmed by switches on the front panel.
Apple II (1977): The Apple Il was a fully assembled home computer in an attractive case, complete with keyboard, connection to a TV screen, color, memory to 64Kb, and BASIC interpreter. The machine was to launch the personal computer revolution and vault its founders, Steve Wozniak and Steve Jobs, from garage to glory.
IBM PC (1981): IBM was neither first nor technologically innovative, but their announcement put the personal computer on the desks of America's business people, just as Apple had put the computer in the home. By 1985 IBM had manufactured its three millionth PC, and had spawned an entire industry in the process.
Apple Macintosh (1984): The Macintosh was far from an instant success, but once Apple got the bugs out and added an internal hard disk, laser printer, and expanded memory, the machine took off.

It's ease of use and graphical interface offered an entirely different perspective on computing.
The PC Today (1996): The march of technology is relentless and astounding and todays PC runs rings around its predecessor. Today you can buy a Pentium processor with 16MB of RAM, a 500MB hard drive, a 15-inch super-VGA monitor, a quad-speed CD-ROM, and a sound card, all for approximately $2,000.