Arduino Open-source Electronics Prototyping
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.

The Arduino board is a cheap, robust i/o board based on the ATmega168. It has 13 digital pins (6 of which allow PWM output) and 6 analog inputs. There are USB, serial, bluetooth, and mini versions,
Build. Schematics and PCB layouts. Assembly instructions: soldering a PCB or on a breadboard (either way, you'll need a bootloader).
The open-source Arduino Environment makes it easy to write code and upload it to the i/o board. It runs on Windows, Mac OS X, and Linux.
Download. From the software page. Learn. From the tutorials.
Adafruit Ethernet (XPort) shield for Arduino kit
"This is a shield for Arduinos that allows use of an XPort, XPort direct, or XPort Direct+ module. Here is a comparison sheet (from GridConnect) about the differences between the three modules.
There is a 3.3V - 250mA power supply on-board for powering the XPort from USB or DC power. Arduino reset button and 6-pin ISP header brought up top for easy development!"