One of the themes that came out of yesterday's excellent Raspberry Jam at Milton Keynes was the very positive overlap between the Arduino and Raspberry Pi communities.
A lot of us Pi enthusiasts also have Arduinos and use them for projects. We're beginning to investigate what the two can do together, and we're seeing a number of projects (like Gordon Henderson's wiringPi) which let people use their Arduino skills and code to drive hardware attached to the Pi.
Some of us are also currently evaluating a Pi shield from Cooking Hacks which allows you to plug in Arduino shields and control them from the Pi.
Of course, not everyone has both computers. If you're trying to decide which one to use Andrew back has written a great comparison on DesignSpark.
If you do have both, there are several ways in which the Pi and Arduino can be connected.
You can use the serial ports. You need to remember to insert a level shifter to convert between the Pi's 3.3 volts and the Arduino's 5 volts, and you need to write programs to send and parse text at each end of the wire.
For most applications I prefer using I2C or SPI. You still need a level shifter, but the software is pretty straightforward. The standard Arduino examples include a sketch that makes the Arduino act as an I2C slave device, which you can then control from the Pi.
Since you decide on the address the Arduino uses on the I2C bus, you can (in theory) control over a hundred Arduinos from a single Pi! I2C is fairly slow, and if you need to send a lot of information to or from the Arduino then SPI might be a better solution.
One combination that I want to explore when I have time (ha!) links the Pi and the Teensy 3. If you haven't come across one before, the Teensy is an Arduino-compatible computer with a very small form factor, and the Teensy 3 has a powerful ARM processor while maintaining a high degree of Arduino library compatibility.
It has 16K of RAM - not much by Pi standards, but eight times the memory of a standard Arduino - and it has a much more powerful processor. Together the Pi and the Teensy 3.0 should form an awesome combination.
