I came across this neat little board the other day:
The Bridge looks like an Arduino shield and connects to the GPIO pins on your Raspberry Pi. After setting up the software you can then hook up Arduino shields to your RPi.
This obviously opens up all sort sof possibilities as there are an array of Arduino shields out there that are capable of some great functionality.
This is also open up the Raspberry Pi for building sensor projects, something I will be exploring in latest posts.