Xbee Shield
The Xbee shield allows an Arduino board to communicate wirelessly using Zigbee. The module can communicate up to 100 feet indoors or 300 feet outdoors (with line-of-sight). 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.
The Xbee shield was created in collaboration with Libelium, who developed it for use in their SquidBee motes (used for creating sensor networks).
Adafruit Servo/Stepper/DC Motor shield
A shield that can control 2 hobby servos and up to 2 unipolar/bipolar stepper motors or 4 bi-directional DC motors.
Battery Shield
A shield from Liquidware that connects to the back of the Arduino, with a USB-rechargable lithium ion battery that can power an Arduino for 14-28 hours depending on the circuit
Liquidware TouchShield
OLED touch screen shield.
Adafruit Wave shield
Plays any size 22KHz audio files from an SD memory card for music, effects and interactive sound art
Adafruit GPS & Datalogging shield
Connects up a GPS module and can log location, time/date as well as sensor data to an SD memory flash card.
Adafruit XPort/Ethernet shield
Allows use of an XPort module for connecting to the Internet as a client or server.