The Adafruit Motor/Stepper/Servo Shield gives you excellent flexibility in ways to create motion with your Freeduino SB or Arduino microcontroller!
This shield adds USB Host functionality to popular Arduino platform.
This Adafruit GPS shield turns your Arduino into a GPS location datalogger! (GPS & SDCard not included)
The HV Rescue Shield 2 is a high voltage parallel mode fuse programmer for Atmel AVR microcontrollers.
Want a tidy AVR-ISP compatible programmer that also acts as a USB to TTL converter? Pololu's Orangutan USB AVR programmer fits the bill!
The LoL Shield is a build-it-yourself kit that creates a charlieplexed LED matrix for the Arduino