The Raspberry Pi is a computer the size of a credit card. To use it, you need a keyboard and mouse, a monitor, and an SD card with Linux. The Raspberry Pi is powered by USB. The creator, Eben Upton at the University of Cambridge in Britain, is surprised at how popular the Pi has become in the few short months it's been available.
From John Bigg's New York Times story about Eben and his computer:
The Raspberry Pi Foundation began selling the computers in February of last year. They soon could not keep them in stock.
“We honestly were thinking of this as a 1,000- to 5,000-unit opportunity,” Mr. Upton said. “The thing we didn’t anticipate was this whole other market of technically competent adults who wanted to use it. We’re selling to hobbyists.”
Mr. Upton said he was “blown away” by the reception the Pi had gotten online.
“I’m not aware of a company that has gone from a standing start to a million in a year,” he said. “It’s quite a wild ride. I don’t get a lot of sleep at the moment.”