http://www.raspberrypi.org/
The Pi is pretty cool, but sadly it has been hyped for a couple of years and a limited run only shipped earlier this year to people on a waiting list. Some are still waiting, myself included. I gave up.
The $25 and $35 price tag has caused a huge supply/demand problem.
There are alternatives for slightly more outlay, some are better in many ways(particularly of you are developing hardware) like the BeagleBoard