Phil Howard runs a website called Gadgetoid and runs a Raspberry Pi blog on a Pi itself.
He’s written an extensive guide on creating something called a Pico PiDuino. It’s a little Arduino-compatible device with an ATmega 328 at it’s core. The tutorial thus far takes you through creating your own and connecting it up to your Pi, just to confirm that it works.
Future tutorials will show you what you can do with it and actually make LEDs flash n stuff 🙂
It’s a really great tutorial and, if you’ve got a hankering for doing some hacking and electronics stuff, give it a go.