Make:

Massimo Banzi writes over at the Arduino blog:

After a couple of years of prototypes, tests and announcements, finally we have produced an ethernet shield for Arduino. This module gives Arduino the ability to open connections to other Internet hosts or behave like a server, for example a simple web server. We like this design because it uses the w5100 chip from WizNet that implements the whole IP stack in itself providing up to 4 sockets simultaneously. I believe this will provide Arduino enthusiasts around the world with a platfrom that is initially simple to use but with room for growth as the knowledge of the user increases. From the Internet of Things to the Internet of Arduinos, happy tinkering.

More:

• Ethduino
• HOW TO - Make plants talk! They'll Twitter you when they need to be watered (and more)...
• The Arduino ethernet board
  

From the Maker Shed:

Arduino Diecimila

XPortShield for Arduino Kit (not the same one as mentioned above)

Getting Started with Arduino- This valuable little book offers a thorough introduction to the open

source electronics prototyping platform that's taking the design and hobbyist world by storm. Getting Started with Arduino gives you lots of ideas for projects and helps you get going on them right away. To use the introductory examples in this book, all you need is a USB Arduino, USB A-B cable, and an LED. By Massimo Banzi, co-founder of the Arduino Project.

Making Things Talk - Programming microcontrollers used to require an expensive development environment costing thousands of dollars and requiring professional electrical engineering expertise. Open-source physical computing platforms with simple i/o boards and development environments have led to new options for hobbyists, hackers, and makers. This book contains a series of projects that teach you what you need to know to get your creations talking to each other, connecting to the web, and forming networks of smart devices.