Written by Massimo Banzi (the cofounder of the Arduino Project) this nice little handbook walks you through the opensource hardware/software project that is Arduino!
Hear of the Arduino / Freeduino yet? It's an opensource hardware platform for the Atmel ATMega168 microcontroller plus the programming system that runs under Java for Windows/Linux/Mac. It's gathering a huge following being opensource and hackerfriendly.
If you're new to the scene you may want to check out this cool little handbook by Massimo Banzi (the cofounder of the Arduino Project). Getting Started with Arduino gives you a solid introduction to this platform so you'll soon be up and running with your own projects!