The Raspberry Pi (in case you’ve been living under a rock for the last six-months) is a cheap ($25) ARM Linux computer, primarily designed to be used to help teach kids to learn programming & computer science.
It’s also makes a pretty nice alternative to something like a BeagleBone, if you’re looking to play with some embedded computing technology. Whilst it’s less powerful than BeagleBone (and has less GPIO connectivity), Raspberry Pi does have a few advantages. It’s cheap, and it has an HDMI output – meaning that you can interact with it directly, without the need to ssh in from another computer.
The whole ethos of the Raspberry Pi foundation is to make the device as easy to use as possible: to encourage children (and adults!) to play with it, and learn to program. As with Arduino though, this user-friendly layer doesn’t mean that you can’t get your hands a little dirty and see what’s going on underneath.