On March 5, 1981, Sinclair Research launched the ZX81 home computer in the U.K. (It was also known as the Timex-Sinclair TS1000 in the U.S.) It came with just one kilobyte of memory, and was a self-contained unit with a rather crappy keyboard. The keyboard didn't have moving key switches; instead it used membrane buttons similar to those often used on microwave ovens.

Despite its limitations, the ZX81 was a revolution, because it cost just £49.95 in the U.K.—massively cheaper than anything else on the market. It was also available in normal retail stores, rather than specialty computer shops.

It really was the people's computer, and for many it was their introduction to home computing and computer programming. Incidentally, at that cheap price, it was a kit you assembled at home (a soldering iron was required). You'd have to pay an extra £20 if you wanted a pre-assembled unit. In the U.S., the fully-assembled unit cost $149.95.

The ZX81 was also expandable. You could upgrade it from its RAM using an external cartridge to bring it up to 16k—making it vastly more usable for real work. If you needed to store programs, you saved them on cassette tapes using a tape recorder. This was a finicky process, as you had to fiddle with the volume to get things just right...but for the price, it was unbeatable.

The ZX81/TS1000 sold millions, despite its limitations. Although it didn't take over the computing world, its serious focus on retail price made it a common computer in the early home computing market. (My family had one!) It was literally a fraction of the price of competing systems. Here's a detailed remembrance of the ZX81, showing some of what it could (and could not) do:

Here's more detail from that interview, including a discussion of the "wobbly" RAM pack:

If you want to go deeper, read this 1982 interview with Clive Sinclair, watch this long interview with Sinclair employee Ruth Bramley. The ZX81 Wikipedia page is also quite solid.