If you already have a Raspberry Pi (I'm hoping you don't!) or you know all about them, then you can skip this section ...
RPis are essentially credit-card sized computers.
They're intended for educational purposes, but for us they make a great basis for side-projects!
RPis are particularly great for embedded projects; the possible RPi uses seem endless when the GPIO pins are considered.
There are some great 'Getting Started' articles out there, including engagdet's and Raspberry Pi's. These both include the items that you'll need before getting started.
To get going, you'll also need to load an OS: Raspbian is the official open-source linux-based RPi OS, which is pre-installed with Python, Scratch, Sonic Pi, Java, Mathematica and more.
Once up and running, you basically have a full Linux box running Debian.