In the time before the PC there were many pretenders to the throne. IBM, Hewlett Packard and Digital Equipment Corporation all made attempts at lower priced machines to compete with the big iron of the day.
Probably the most significant among these was the DEC PDP-8. This "minicomputer" was, in many ways, fairly prototypical of the soon to be introduced microcomputers. It was also the first computer to ever be called a "minicomputer."
The PDP-8 (Programmable Data Processor) was introduced by DEC in 1965, nearly ten years before "Personal Computer" revolution really began. It was sold outright, which was unusual in an era when companies like IBM would only lease their equipment, and the price was very reasonable at about $19,000.
Other early machines included the Scelbi 8-H which was sold as a kit as early as 1973. This machine didn't use a microprocessor, instead using discreet logic for all of its operations. The computer didn't experience very much success and the company eventually transitioned from hardware to software.
The heart of a Microcomputer is, of course, the Microprocessor. That particular device was created by Intel in 1971 as the 4004 microprocessor chip. This 4 bit unit was used mainly in calculators but was the launching point for a product line that would eventually revolutionize computing. The 4004's successor, the 8008, was an 8 bit chip that was introduced in 1972 and ran considerably faster. It was used in many applications, one of which was an obscure kit computer introduced by Radio Electronics magazine in July of 1974.
In that same year, Intel introduced the 8080 microprocessor which was selected to be at the heart of the MITS Altair 8800. That chip and that computer marked the beginning of the popularity of Personal Computers, although it should be noted that the notion of personal computers and the hobbyists that built them were well into the hobby 10 years earlier.