Need the stability of a physical Ethernet connection to build a simple, embedded web server? Send Tweets? Check email? Need to bring power over Ethernet to your Arduino? The Arduino Ethernet shield can do all these things and more! Each Ethernet shield is ready to go out of the box (the Arduino Ethernet library will take care of DHCP), and its MAC address can be found on a sticker on the board itself.
For example projects, documentation, and help getting started, see http://arduino.cc/en/Main/ArduinoEthernetShield