Introduction at Arduino site: http://arduino.cc/en/Main/ArduinoEthernetShield

The new Arduino Ethernet Shield allows an Arduino board to connect to the internet. It is based on the Wiznet W5100 ethernet chip. The Wiznet W5100 provides a network (IP) stack capable of both TCP and UDP. It supports up to four simultaneous socket connections. Use the Ethernet library to write sketches which connect to the internet using the shield.

This is the latest version.

Available features:

•    Micro-SD card connector
•    On board reset controller
•    Compatible with the Arduino UNO, Arduino
•    Duemilanove and Mega
•    NOW Compatible with the PoE Module.

Base kit to extend your Arduino MEGA with your own wired circuit.

Base kit to extend your Arduino with your own wired circuit.

This shield adds wireless capabilities to Arduino board allowing it to communicate wirelessly using Zigbee. It can be used as a serial / USB replacement or you can put it into a command mode and configure it for a variety of broadcast and mesh networking options.

Does not include XBee module! See our XBee 2.5 module.

Here is a design for a full-featured motor shield that will be able to power many simple to medium-complexity projects. Unassembled kit with all parts nessessary.

2 connections for 5V 'hobby' servos connected to the Arduino's high-resolution dedicated timer. 4 H-Bridges. Up to 4 bi-directional DC motors with individual 8-bit speed selection. 

The ScrewShield is a “wing-format” shield that extends the Arduino pins to sturdy, secure, and dependable screw terminal blocks. (You even get a few bonus terminals for extra GND and power!)

The wing design allows you to extend just one or both sides (analog & digital) of the Arduino, and still access the jumpers, LEDs, and buttons on the Arduino.

Adding quality audio to an electronic project is surprisingly difficult. Here is a shield for Arduinos that solves this problem. It can play up to 22KHz, 12bit uncompressed audio files of any length. It's low cost, available as an easy-to-make kit.

Audio files are read off of an SD/MMC card, which are available at nearly any store. Volume can be controlled with the onboard thumbwheel potentiometer.

The MIDI Shield board gives your Arduino access to the powerful MIDI communication protocol, so you can control synthesizers, sequencers, and other musical devices. The MIDI protocol shares many similarities with standard asynchronous serial interfaces, so you can use the UART pins of your Arduino to send and receive MIDI's event messages.

This shield gives the Arduino CAN-Bus capability. It uses the Microchip MCP2515 CAN controller with MCP2551 CAN transceiver. CAN connection is via a standard 9-way sub-D for use with OBDII cable.