The Raspberry Pi is a tiny single-board device that plugs into a computer monitor or TV, and uses a standard keyboard and mouse.
It has all the capabilities of a desktop: going online; playing high-definition video and games; and making spreadsheets and word-processing.
Project founder Eben Upton said: “We’re calling it, we’re the best-selling UK computer ever.”
Developed in Cambridge and manufactured in Pencoed, South Wales, the computer was originally designed to help introduce people to programming and is now used in education and industry.
Mr Upton said: “The two main things that people do with their Pi are use it as a PC replacement or use it as an embedded computer.”
“The Pi 3 is doubling down on both those things rather than going looking for new things to do.”
Now, as children and hobbyists quickly learn how to use the cheap gadget, it has reportedly become the most popular computer built in Britain.
According to Mr Upton, Raspberry Pi has sold more than eight million units, overtaking Amstrad PCW’s record.
With its “entry-level” Model A+ costing 20 US dollars (£14), Raspberry Pi tries to make computing accessible.
To celebrate its fourth “birthday”, the company has launched a new model, the Raspberry Pi 3 - containing a faster 64-bit processor and built-in wi-fi and Bluetooth.
It costs 35 US dollars (£30) and “a few hundred thousand” units are being sold online at element14, RS Components and other resellers.
The Raspberry Pi 3’s 64-bit processor also means performance is 50% better than the previous model.
The wi-fi and Bluetooth additions enable users to link their devices, such as as smartphones and tablets, together in a single hub.
Mr Upton said: “This is the first Pi you can stick behind your TV and completely forget about.”
Beginning with just “a handful of volunteers”, Raspberry Pi has grown rapidly - a model was recently sent to the International Space Station with Tim Peake and it is widely used in schools.
The company is involved in various education programmes, including the Pi Foundation’s merger with the Code Club initiative, a voluntary organisation teaching children aged 9-11 how to code, and it works with Google to train teachers through a programme called Picademy.
Mr Upton said: “With 9-11-year-olds, we are seeing a lot of people get excited about it at that level.”