The computers are no-nonsense in design and presentation. There are USB ports for a keyboard and mouse, an Ethernet port for the Internet, a slot for a memory card and a port to connect the computer to a TV. The operating software is a form of the open-source Linux system.
Production was passed over to two British firms which now pay a royalty for each Pi sold. On the first day alone, 100,000 were sold.
By October 2012, 1 million had been manufactured in Britain, production having transferred from Asia. The biggest market is the US, where 30 percent have gone.
About 20 percent are in Britain, another 30 percent in the rest of Europe — mostly Germany — while the rest have mostly gone to Japan and South Korea.
More than 400,000 of the 2.5 million sold are thought to be in the hands of children, while most of them go to adult hobbyists.
So far the Pi has been used by hobbyists for everything from creating a 3D scanner and calibrating temperatures for home brewing to a laddish invention which orders a pizza on the Internet via the touch of one outsized button.