Altair 8800
Today, December 18, is a very important day in computer history for those of us who grew up with with the microcomputer revolution.
On this day in 1974, the MITS company put the Altair 8800 microcomputer on sale -- ads were printed in the January 1975 issue of Popular Electronics. The kit came with all the parts you needed to put together your own microcomputer for $397 (or $495, assembled).
The Altair didn't come with a keyboard, monitor, or any storage devices. It did have 256 bytes of RAM memory and some front panel switches and LED's that you could program to blink. You programmed it by setting the switches to a binary value, and entering that 8-bit instruction or data.
The Altair used the Intel 8008 8-bit microprocessor, which had been released in 1972 and ran at a blazing200kHz. The normal price for the 8008 was $360, but apparently the founders of MITS got a deal (around $75) for their kit.
The computer was expandable by plugging in cards to a backplane, which became known as the S-100 bus. Eventually there were add-ons such as cassette tape storage, RS-232c interfaces, keyboards.
Bill Gates and Paul Allen wrote the first BASIC language interpreter for the Altair. That success led to Micro-Soft (they had a dash in the name back then).
The product was a huge success. The Altair sold 4000 units in the first 3 months. It's release led to the first home computers: The PET, the Apple I, and the TRS-80 Model I, among others.