hacknmod.com — Top 40 Arduino Projects.

en.wikipedia.org — Arduino is a tool for the design and development of embedded computer systems, consisting of a simple open hardware design for a single-board microcontroller, with embedded I/O support and a standard programming language.[1]

cws.gtc.edu — Learning Objects for Electronics

makezine.com — MAKE Magazine brings the do-it-yourself mindset to all the technology in your life. MAKE is loaded with exciting projects that help you make the most of your technology at home and away from home. We celebrate your right to tweak, hack, and bend any technology to your own will.

arduino.cc — Arduino is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. It's intended for artists, designers, hobbyists, and anyone interested in creating interactive objects or environments.

palmavr.sourceforge.net — Atmega fuse calculator - Select your chipset and options to see the hex representation of your fuse settings.

atmel.com — The full datasheet for the Atmega 1280, including pinouts.  PDF Format.

moderndevice.com — Arduino and accessories supplier  

fritzing.org — A site dedicated to helping users with PCB layout and connecting with manufacturers for production.