Raspberry Pi is a credit card size computer running Linux. It features ARM11 microcontroller, GPU, 256MB of RAM and bunch of I/O interfaces like RCA video, audio, USN, LAN, HDMI, SD. And of course there are couple decades of digital I.O pins that can be useful in various hacking experiments. Tedbot wrote a tutorial on how to control I/O pins in Arduino style.
He tried three methods of doing this – Python, Bash and C. Python method require Rpi GPIO module so you could write simple IO commands in Python interpreter. In order to program pins in Arduino style there is a nice library called WiringPi. And finally in order to access pins using Bash all you have to do is to treat pins as files and echo in/out to perform read/write.